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You're right that the coinbase and a growing number of pre-requisite transactions will not be valid on the 110 chain. At the beginning, all non-spam transactions, essentially, will be valid on either chain. And, for a while, most transactions will need to worry about which chain they land on. I believe that does answer your question.
Yeah, that window is 100 blocks unless there is overtake risk (40%+ basically, though if some pools do not pay attention, it could be done with less; 34% selfish BIP-110 versus a naive 66% non-110 - though that is unlikely, it is not like this is a stealth op.)
So this is what happens:
The exact constraint for "worry" is that within the first 100 blocks post factual fork, when forked coinbases are unspendable, BIP-110 has between 34% and 66% of realized hash power. If it doesn't have that, it's either succeeded or failed and there is nothing to worry about. I think that the whole thing balances on whether you think that this 34%-66% scenario is likely. I don't see it right now, but maybe I am naive.
A coinbase from the non-BIP-110 chaintip post-fork will also not land on BIP-110 because those coins don't exist in that chain. So you're not just aggressively excluding spammers, you're aggressively excluding everyone that does not share your point of view 100%.
For example, I dislike the spam and meta protocols in general, and I do think that there's an inherent weakness in Bitcoin that enables it. But BIP-110 is a poorly designed solution. Not like taproot that made a couple of poor auxiliary choices, but intrinsically dubious choices, with as only mitigation the temporary nature: Bitcoin would be in a miserably designed state for a year. That prevents adaption of the proposal, and the root cause is ultimately rushed execution in the development process: it was rushed because of what appears to be an emotional foundation of arbitrary deadlines (that weren't met, so it was pointless to begin with.)
High level: The day that some bubble on twitter is going to rule Bitcoin through force is the end of liberty in Bitcoin. If a subgroup (explicitly aimed for with 55% MASF that is likely to fail in the remaining few weeks, and a minority built in through the UASF side) can dominate consensus by force rather than by building it, then Bitcoin-the-tool-for-resistance is dead. Because this would mean that all a hostile state actor, or - perhaps even worse - some self-sovereign sociopath or puppet of one, would have to do is gaslight twitter to control Bitcoin.
PS: you didn't answer my question.