I'll get to a couple of questions now and then come back later for the
others above.

Tail draggers - I think it does them like one would expect: Ground loops, etc.

Spins - In normal cruise flight the aerodynamics are 'linear' (lines
of data are mostly straight). In a spin, the angle of attack is high
and the wing is stalled, so now things are 'non-linear' (curves and
dips, not much is straight). This makes modeling spins hard relative
to cruise flight. Why isn't more research done? It's a
good question, but I don't have an answer for that one.

Michael