... It was the surprise at that point that gave Harry the advantage against Thorny at the Shedd ...
I'm reminded of story I heard about a very-senior jiu-jitsu sensei who was having a Very Bad Day, he was angry about I-don't-remember-what.
Somebody unaware of his mental state managed to walk up behind him, clap their hands over his eyes meaning to say "
guess who" but before the first syllable was out of their mouth they were airborne in a the kind of throw that ends in a full-body slam onto the floor with both hands tangled & unable to slap a breakfall
at all.
Also reminded of Harry triggering one of Eb's defense-contingencies.
When you "get surprise" on someone, best make sure they aren't much, much better than you.
I absolutely do not believe utterly-new-to-Soulfire Harry could have so utterly manhandled any Angelic being... it is beyond "implausible."
(fwiw: the sensei mentioned above (once their "foe" was in front of them instead of behind, i.e. could *see* them) was able to alter the throw to be setting-them-down-upon-their-feet)
... It's logical that Namshiel would want a fortress like Merlin's Castle to work out of, he may have wanted it for a long time. He finally has a host, i.e. Marcone, who actually had the financial backing and means of making it happen. Reread the bit in Twelve Months where Harry does the "Wizard of Oz" trick using the leylines under the castle as a power source, when he warns the vanilla demonstrators. That's why Namshiel wanted that particular location, the leylines that just happen to cross where Harry's basement lab was located. Oh another reason, remember Lea's garden is also located at that particular location. Result there is no reason to conclude that Namshiel is working or attempting to work on the same side as Harry based on the evidence presented.
Consider your accounting of Thorned Namshiel's motivations and desires here. He "finally has a host" who can get him the castle/base/HQ/lair/whatever that he has long dreamed of.
Consider the scene where Harry demands the castle. Does Namshiel just
allow Marcone to give up that long-sought prize??!? Once again, this bursts the bonds of credulity.
This was all maneuvering to get the castle into the Well-Warden's possession.