1. I'm not sure about remapping 3rd, 4th etc mouse buttons, but you might try opening configuration -> keyboard, selecting a cell, then clicking that button. Otherwise there's the hard way of binding it to a key you don't often use, then using a third-party program to remap mouse4 to that key.

2. If a target is selected, you can press enter/confirm to confirm the target, which will change the indicator and show a little sword icon if you're in active mode. If you see the little sword icon, you will autoattack when in range I believe. Sorry, I'm used to the UI at this point so it goes unnoticed most of the time.

3. Yes! I believe the default is F, but it may be what I have it set to, which is Z. You can remap many functions to Crtl/Alt/Shift + [most keys] in Configuration -> Keyboard.