Fine.
Not fine. Why does the character who reacts last - both in the fiction* and at the table - get to resolve first?
* - which must be true as you can't react to something that has yet to happen, you can only pro-act; reaction by definition cannot occur until there's something to react to.
Timing.
Again, you're ignoring the part where I am saying that the order of events does not have to be the same for the characters as the players. Your view is far too strict regarding the turn structure and the designation of the action as a "reaction".
Part of reaction often is being ready for something. Anticipating or expecting it and then responding once it happens. This is what I've been saying.
If you simply don't enforce the turn structure onto the characters in the fiction, then it all can flow perfectly fine in the fiction, and perfectly fine at the table.
It's this perceived need to have them match that causes the issue. That desire to have them match is not a need, but rather a preference.
The answer to that bolded bit is the same reason why you can't learn to safely text & drive a thousand plus pound vehicle moving at potentially deadly speeds.
Humans are bad at multitasking. We need to go through
full context switching rather than a lower overhead multithreaded conscious thought. I
t's not a thing our brains evolved to be capable of doing. Taking that to the question of why forcing the GM to engage in context switching is problematic you just need to factor in that
most GMs are human. but 5e is designed in a way that simply shrugs it off.
I'm unsure if you're talking about the characters here or the GM.
If you're talking about the characters and their ability to track multiple factors in a hectic environment such as combat, I would agree that it is incredibly difficult. But it's also something humans have always done and continue to do.
If instead you're tlking about the GM, I don't think it's too much to ask a GM to allow reactions. I handle them fine when I GM, and every other GM I've seen handles them fine. If a particular GM actually struggled with them for some cognitive reason, then sure, he should limit or ban them. But I don't think that the rules need to be changed to meet that specific GM's needs.