Bincoin is not Bitcoin core client it's a protocol. So if transaction is valid according the rules it's valid transaction. Even now you can submit a non-standard transaction to miners and it will be accepted into block.
I agree. I was just answering your question on how would a miner know which ones are signed by the adversary. Miners are usually (I think they had an agreement in the past) running Bitcoin Core so they might not just pick up the one with highest fee, but discard it as it came after the first one.
Of course you shouldn't count on this.
Blocks will be full in the future too. Even with level 2 solutions in place. If blocks would have empty space then on chain fees would be near zero. And how would miners get paid at this case when block subsidy become 0? So 10 min. it's for highest paying transactions only even in the future. 
Now this is a prediction game. I can't really answer this as no one can. What is exactly near zero, that is the question.
Block subsidy might get near zero, depending on how near we are talking about here, but it will not be zero as long as there are any transactions that pay the fees.
If the subsidy is too low, then there will be less miners. It is a self-correcting mechanism.
But then again, who knows...
Many things will be very different from now by that time. Lower (or none) block reward, off chain transactions, high adoption (hopefully), etc.