I can't comment first hand on alot of your questions. However, the consensus on here appears to be that FAA licences are generally less costly to obtain and slightly easier in the sense that a JAA ATPL requires taking 14 exams and doing compulsory consolidation course of two lots of two weeks. To get a "frozen ATPL requires the exams, CPL (25 hours), Multi rating (5 hours) and IR (55 hours) + MCC (not sure how many hours this is) - all these figures are of course minimum requirements.
You can convert an ICAO ATPL to a JAA ATPL. This requires you to undergo some theoretical knowledge instruction and pass the 14 exams, do a multi pilot type rating course and pass the Skills test. You can avoid some of this if you have > 1500 hours. The JAA requirements are set out in the CAA document "LASORS 2008, Section G" (Google that and you will get the relevant section).
If you definitely need the JAA ATPL and as there are a number of JAA training organisations in the US, I would have thought that this route is worth a look. There was some question that with the take over of FCL by EASA the ability to train for JAA licences in the US may be cut off ( I can't remember where I read this though!).