My assumption is that the kind of person who would trade in monkey jpgs isn't informed enough know any of those details about where on chain it is stored or going to take any path other than the cheapest possible to the next block.

1MB base and 3MB witness. I think they will end up split on chain once base is full and witness is empty.

2 problems with filters that combine to keep block validation filters from being taken seriously.

1 is they are just way harder than you think to get right if you've never had to write a regex.

2 is that any filter on valid blocks must allow all existing blocks or fork and rollback. we don't know what our adversary has cooked up to sneak through the filter till it hits mempool, or in your case gets in a block. That is too late to get the fixed filter in place.