Uneducated guesses:

to 1: The Euronorm crashworthiness standards are superior to the (old) FRA standards. And, in my understanding, mixed operation is permitted. The report states that the modifications of a "standard" European high speed train are very small (around 1%); modifications beyond that are based on customer's demands, and not caused by regulation.

to 2: This is the case. Good example: Caltrain getting KISSes by Stadler. The modifications demanded by Caltrain are affecting the base concept only in a limited way. As you mention DMUs, there is not that much of a difference between the FLIRTs built for that Texas agency and the Flirts currently built for Greater Anglia or Wales (actually, the latter have more deviation from the standard design, as they are adapted to high-level platforms), or for the Swiss SOB (with the difference that those don't have a diesel power unit, because they don't need one). The situation is similar for the product families built by the other major European manufacturers (aka Alstom, Bombardier, Siemens). Concerning the "buy america" factor, it could be that, if ordered in sufficient quantities, the standard European DMUs would cost less by avoiding the "buy america" regulations, and find other financing, than following the "buy america" rules.

to 3: I don't know, but they may not qualify because they are built to follow the old FRA standards, and that makes me guess that they do not have any CEM elements built in. That means that there would be a considerable need for a redesign.