1) A warning from EVELYN is usually the result of a condition that is always true (or false). This can be either due to programmer error or intentional, therefor the warning. If the generated code looks good to you you can either ignore it, suppress it (with command line option or pragma) or rewrite the code.

2) I'm afraid there is no list of errors/warnings outside the source code of the compiler.
SDCCerr.c
SDCCerr.h

3) The interrupt keyword and its number are mandatory because that is what tells the compiler which vector to fill. The using keyword is allowed, but not necessary since it behaves like a toplevel in the calling tree anyway.

Maarten