All of those things that you listed as not having defense rolls DO have some form of minimum value to determine success, if not an (unrolled) resistance value (ex. zone borders for movement rolls).
The same is not true for blocks.
Actually, if you're going to go that route, blocks
also have a minimum value to determine success. If you try to establish a block and roll zero or less - you fail.
The advantage in the first exchange perpetuates into subsequent exchanges. In other words, it persists, or is persistent.
Again, different definitions in play - I look at the words "persistent advantage" and read "an advantage that is difficult to remove", rather than "an advantage that has effects in subsequent exchanges." In fact, I would hold that any advantage that does
not have any effect on subsequent exchanges, wasn't an advantage - such as the initial grappler under your interpretation, who is in fact
penalized for his aspect tag (by allowing the counter-grapple to happen).
The problem is that a block with low strength is essentially nonexistent.
This depends on what you're blocking. In the case of a grapple, in particular, even a low block strength can make a large difference - for an example from a game I was in, a mere strength 2 grapple caused a caster to fail to fully control a spell (because the block reduces the margin of success). Likewise, a low strength block can hamper someone's attempts to flee, making it easier to catch up.
A good example here is two runners, a slow one who got a head start, and a fast one who's trying to catch the other. The second one, each exchange, sprints to the zone the first one is in - and uses overflow to establish a block against further movement. In a couple of exchanges, movement will no longer be an option for the slower one, and he'll have to face his pursuer, try some sort of clever maneuver, or otherwise find a way to make the situation stop being a straight race. Low strength blocks are only useless if they're specifically against attacks and are too far below your normal defenses.
Also, blocks are never defended against under normal circumstances. They are established, then people try to break them. So where a block normally replaces a defence roll, against a block it creates one (the way you read the rules, at least).
This is a bit of a problem.
I don't see the problem - but if you do, see my previous statement on disallowing blocks to block blocks. Doing so is RAW legal, just (imo) not RAW required. But out of curiosity - suppose that you just took a consequence in combat, and you're fighting grapple-monster-of-doom - what sort of game mechanic would you use for taking action to try and prevent it from grabbing you?