1) The commercial checkride is mostly airwork. There are some new
maneuvers to be done. The oral is a bit more intense than the private,
mostly with regard to aircraft systems (be ready to answer the output
of the alternator in amps and the capacity of the battery).
2) The FAA's position is that the "solo" cross country must truely be
solo. Exceptions are made for pets.
3) You can't do much with the commercial ticket. You can work at a site
seeing operation or you can go after your CFI. You can also work for a
121/135 operation. Actually taking people places for money is very
different and requires 135.
4) The second class medical is almost identical to the third
5) If you add a seaplane rating it will not automatically be commercial
but it will not be very difficult to take the commercial add-on vs the
private add-on sea checkride. As I recall the only difference is that
commercial must demostrate docing.
6) Yes, if you are a non-citizen your first stop must be to register
with TSA. That paperwork will take a bit of time. AOPA has the best
info on this. Once you decide on a CFI or flight school you will need
to work with them because your TSA approval is LOCKED to them. If you
change CFIs or flight school you must reapply with TSA, get a new
background check, pay another $130 and reshow your passport. The CFI
and FBO must already be registered as a TSA training facility.

-Robert, CFI land & sea