Yes, of course this is all possible.

I was just wondering if it is possible to do one simple include in the place where I want to see the code of my library in memory and use the functions without doing any definitions of the variables and the answer is that you may have convenience of use or control of there the code is placed in memory or convenience of pasting library code in a form of one piece "include" but not both if you want to use variables before the "include".

In most cases this is not a big deal - just put all includes on the top, unless you want your main cycle and interrupts work as fast as possible, then you start moving seldom used code down, play with clearwdt and so on and then you have to split includes or just copy pieces of it's content in two separate places.

Again, this is not a big deal, only a matter of some convenience of having own library functions