One of the worst problems you encounter with misaligned mirrors is inconsitent cutting power and wrong cutting angles.
1. Yes, as you want the laser beam to go down exactly 90° and into the center of the lens.
2. Yes and no. You want the beam to stay in the same spot on the first and second mirror when you move the axis from mim to max.
The 3. mirror can be out of alignment if the other two are fine, in this case you adjust the mount of the mirror/head to get the beam center.
In any case you don't want the beam to wander on the 3rd mirror.
3. No problem at all hitting the lens alsways dead center if all mirrors are aligned properly.
Good results come from good hardware, even cheap hardware can be aligned properly if you take the time to do so.
The reward is that you have even focus and power wherever your head is working.