A few misconceptions here....ca_flyer has it right (should do with a name like that
)
You can still add FAA ratings to the FAA certicate based on your JAR one. These are endorsed "US Test Passed" on the back. I added IR and ME to my one when I had it. Eventually I went for the CPL without ever doing a FAA PPL.
A BFR IS indeed required, and every two years afterwards. The FAA certificate is valid as long as the JAR one is, the "temporary" comes from the bit of paper the FSDO will give you which is valid for 4 months. By month 4 the nice credit card one should come through your post box, if not you must contact the FAA and get them to send it.
No test is required, no exams are required.
FAA Medical is NOT required if the foreign (JAR) medical is valid.
On a "based on" FAA certificate you are restricted to the privileges of your JAR licence, so if you have no NQ, then no night flying. Some of these restrictions can be "undone" by passing the US test (IR for example). There is no NQ in FAA land though, the FAA PPL includes night flying.