Help me choose a suitable Scheme implementation
You may want to take a look at Racket. Its documentation and IDE are pretty good, it's JIT-compiled, and it supports a variety of languages including R6RS. The only requirement of yours that I'm not sure Racket fulfills is being lightweight—it's a matter of perspective whether that's true for it or not.
If you don't care about the heaps of functionality that Racket provides, Larceny might warrant consideration as well. It was written by Will Clinger, who was personally involved in the R6RS process. Larceny is one of the most complete implementations of R6RS. Petite Larceny can be tricky to compile, but this may not matter much depending on the platform you're targeting.
Finally, there's Chez Scheme. I have no experience with it; it has a reputation of being an efficient and well-supported Scheme, though. In any case, note that, as far as I can tell, you need the commercial license if you want access to the FFI.
Edit: Added Larceny and Chez Scheme.
while it may not fulfill all of your requirements, i've been having a lot of fun with chicken, which has a vibrant community and lots of libraries for practical applications.