Actually it isn't evidence all that Harry was under qualified ... however the very fact that Harry was able to pull it off says he has the skills and talent to do the job ...

It's really quite necessary to ask, from whose point of view?

Demonreach has its own standards.  Obviously, Harry fulfilled them.
I might speculate:  others know there's an Apocalypse coming, so it seems likely Demonreach knows, too.  Did the entity accept Harry's effort at a lower bar, because it was so critical to get even a mediocre Warden over none at all?  I'll assert it's a reasonable question, to which we have no answer (and very little evidence).

And, of course, the White Council has its standards.  Demonreach (and its Warden) are no part of the White Council; Warden of the Well isn't a Senior Council title, nor yet an unofficial role within the WC (the way "the Blackstaff" is an unofficial role).  The Council isn't even always aware of the Warden.  And yet, they have Opinions!

From the WC perspective, then, Harry's the plumber in the nuke-plant.  Not everyone shares that opinion... Ebenezer thought it was a good idea; Rashid is... mmm... cautiously optimistic?  Others find it an act of hopeless incompetence:  becoming Warden unwittingly, what more proof of incompetence do you need?  And a few, I suspect, still harbor a suspicion that they are seeing Warlock Dresden^(tm) making (yet another) power-play.

... So it's better to leave no one in charge?

I think, from the WC perspective, this is the case.
Because a Warden can release the entities in the island, and there are a lot of horrible (and horrifically powerful) entities down there, gathered over the course of millenia.  I think a bad-guy Warden is just about the worst scenario they can envision, far worse than none at all.

I think they see the Warden's job as being bad enough that nobody "should" want it; that the only reason anybody would want it, is to do  Evil Villain Shenanigans.

... So no, it's not confirmed canon, but it's heavily implied if you follow the logic chain.

Oh, I agree that the implications are there!

In particular, knowing that Lea created her Murdergarden to block Nevernever access -- and killed many attackers -- tells us that many creatures can waltz right across a threshold from the Nevernever; and therefore -- when Lea was herself present in the garden -- so to could the Leanansidhe.

What's more:  when Mab had Lea "on ice," Mab was handling all Lea's "faerie godmother" obligations... so Mab herself was just a step from Harry's lab (this was, by the bye, the timeframe when the "mystery repair" to Little Chicago happened... food for thought ... ).

Another WAG, at one point Bob seduced Mab...

I don't think a spirit could have done that.
Or did you mean the human wizard who "birthed" Bob?

Is this a Word of Jim?  If so, could you direct me to it?

It is, I'm virtually certain (I have read the section repeatedly).
But now I cannot find it.     

... But it is relevant,  neither Bob nor Bonnie can be expected to live in a tiny brain space ...

No, it's irrelevant; "space" (in the 3-dimensional sense) isn't what's involved.

Spirits can fit into arbitrarily-small spaces (through keyholes, under doors), but also expand into very large ones (such as when Bob filled Sue, or the brass lion).

Bob's skull not only fits Bob:  it has room for a guest, as Harry discovered.  What's more, the space didn't seem "cramped" -- several more could likely have visited, if it were needed.

So the inside of "Bob's Skull" offers more room to spirits than the physical space allows; more spirits can fit than brains can.  So too (most likely) the wooden skull that Bonea has; Harry was clear that his wooden skull was cruder and less capacious, but it's still similar, and "good enough."

... Harry carved that wooden scull as a backup home, but if he doesn't know how or cannot make that connection to make that space inside, what's the point ...   

Harry had the original skull available -- and Bob's tutelage -- for years beforehand, to study.  Both the runes & other carvings, but also the enchantments, the spells.  And then had Bob available as a consultant, in prep for constructing the "backup" home.

Moreover:  he did it.  Whatever the minimum enchantments necessary to make a spirit-home skull, Harry managed.

... Another problem with your idea, Bob avoids the Nevernever, so would he be happy with such an arrangement?

Bob avoids the fae, particularly Winter, and most particularly Mab.

AFAIK, the only reason Bob "avoids the Nevernever" is the Fae/Winter/Mab issue.  If the skull connects to a little isolated bubble of "Neverneverness" then Bob can go there and be safe. (***but I have a WAG about this!)

I think it's creating this isolated pocket anchored to the skull, that makes the job more than a simple carving.  Harry could have bought a cheap wooden carved skull from a Día de los Muertos leftover-sale, if it were as simple as all that (2/$35 on Etsy, I just looked).


*** My WAG about Bob & his "fear of Mab:"
1/  I think Bob's "Spiritual Heavyweight" parent was Mab.  I know that Lea is a fan-favorite, but IMHO she's too much of a lightweight power (I know, right?).  Mab -- in charge of the Outer Gates of Creation -- is inestimably more-potent.
2/  I think Mab created a Shadow in the mortal wizard specifically to get a spirit-child (Bob) out of the deal (and if it was Mab, there was a deal!), likely Mab's Shadow providing knowledge/advice to the wizard-host, both as Mab's part of the deal and what Mab wanted in the first place.
3/  I think Mab wanted (and got) a "Spirit of Intellect" that would work closely with powerful human wizards, through the centuries; but one who, ultimately, served Mab's interests.  A deep-cover sleeper-agent, if you will.
4/  I'm unclear if Bob knows he is Mab's agent (and is only pretending he's terrified of her), or if Bob is an unwitting accomplice; or what.  But remember how Mab knew exactly what was going on with "the Parasite" and provided a carefully-crafted countermeasure (did Lea show any awareness whatsoever)?  And how Mab gave Harry an "impossible" order, to kill an unkillable immortal... but knew Harry had access to Bob (who knew the secret).  And how Bob more-or-less LIVED in Harry's lab... right across the Nevernever gate from the murdergarden of Mab's Handmaiden.
Mab could clearly have destroyed Bob any time she wanted to.
Bob's "fear" of Mab is pointless; the question becomes:  is Bob fooled about his danger?  Or is Bob fooling Harry?

... the inside of Bob's skull home is very much a home, more like the TARDIS of DR WHO, bigger inside than outside ...

The physical space is irrelevant, though.  The "space" inside Bob's skull isn't "space" but a connection to (what I suspect is) a demesne within the Nevernever.

... it was a credible threat because Harry was Warden as of Turn Coat, he just didn't know it ...

Harry threatened Mab with prison at the end of Cold Days, I think -- well after TC -- so Harry was very-much aware of the Warden role; but that isn't the point here.

The point is a very-short logic chain:
1/ Harry issued a command to Alfred to be carried out in the event of Harry's death.
2/ If Warden-specific stuff expires when the Warden expires, Mab wouldn't have seen the threat as credible; but she did.
3/ Therefore, at least some of Demonreach's Warden-stuff survives the death of the Warden.

Presumably, it eventually expires (if everything could be on-tap all the time, I presume it would be, and the Island's defenses wouldn't have been so lax at the time Harry became Warden); hence my speculation above, regarding "year and a day" or the like.

Still doesn't follow, Kemmler as Warden still would have had control of the inmates ...

That's our understanding, yes.
I confess:  I don't have track of the WoJ (or canonical cite) which says this.

But AFAIK the inmates are still subject to (as yet unknown) specific rules of Demonreach.  It's in BG, I think, where Harry discovered that Alfred does stuff outside of the Warden's orders/control/perception; I expect that brief reveal will prove important later!

...  Another thing that doesn't fully compute, the island's defenses..  The ones that one the Warden has control of, they weren't in place when Harry took over as Warden.  Alfred was barely holding on and that's why he was so happy to have a caretaker again..  Those Warden controlled defenses remain in control even when Harry isn't on the island.. Are they automatically dropped if Harry died leaving the island vulnerable once more?

I don't think the island drops all those defenses instantly, no:  remember, Mab acted as if Harry's order to Demonreach (to imprison her if she killed him) was a "credible threat."

I suspect it'll be one or more funky bits of pseudo-mystical handwavium -- "a year and a day" or somesuch -- that is the duration for "final orders" of Wardens who die.