Yes, you can. You just have to make sure your function is reentrant. If it accesses no memory beyond its parameters and (non static) local variables, you should be fine. But beware of the library functions you call: some of them are not themselves reentrant and should therefore be avoided.

Yes, you can. You just have to make sure your function is reentrant. If it accesses no memory beyond its parameters and (non static) local variables, you should be fine. If it accesses global variables, hardware registers, or static locals, it is unlikely to be, and only detailed analysis will tell you for sure.

Beware also of the library functions you call: some of them are not themselves reentrant and should therefore be avoided. See for example the manual of strtok(), which explicitly addresses the reentrancy problem.