Since the game does not allow you to bind a mouse button to any functional key (shortcut), in order to physically use some skills with your mouse, the only way seems to be binding your mouse key as one of keyboard key with some 3rd party software. Is using such a software against the rules or am I free to use such?
You are free to bind a mouse key to a keyboard key, because such action changes on-board memory of your mouse, which then, on press, sends a keyboard key to the PC via a USB/PS2 cable (or wirelessly). This is a HARDWARE solution, and there is not a single word in the rules that forbids you from using HARDWARE, there is only a point about SOFTWARE. As for looping mouse keys in a macro to perform more advances tasks like auto-farming, that's afaik illegal.
Don't think mouse/keyboard macros are bannable, I used for many months with no problem, some RU streamers like foxis and neurodrive even stream using these macros.