To answer your last three questions.
1-Most of the bolts just hold bearings in place, only a few actually hold things together.
2-If during assembly all your bearings do not make contact with the rails something is wrong. The only adjustments that gt made are tension, very small adjustments. The bolt holes are meant to be very tight to the bolt, if you drill these out you have ruined the piece and it will most likely be crooked.
You are looking for giant adjustments like eccentric bolts or something. This is not how this works. If you watch what happens for every piece when you insert a rail it might be more clear. Every single piece should flex open to accommodate the rail. This is why the middle bolts on every single rial part should be extremely loose when assembling. You can even go as far as not adding a nut to them.
The adjustments happen only on the bolt with plastic on both sides, I call these the middle bolt/bearing.
When you tighten the middle bolts it is bringing in not moving the middle bearing, it is bringing the other two bearings slightly closer.
I am sorry you are having difficulties but as I have said in the emails there is no secret tightening sequence to get this right. Most people’s assemblies come out really square, If you are having issues the only adjustments possible are the 8 middle bolts on the middle assembly. The down side is most change 2 axis at the same time. The only thing I can suggest is loosen or tighten a bolt and check how it effects your assembly. Put the square on your rails and watch it move. When I need to do any adjustments I spend ten minutes doing just this. Obviously it is much easier not on the rollers.
You ask for a dimension at the end of you post, I am unclear which one you want. I think you are looking for 27.5mm. Squaring tools are not a good solution, you should check your roller to corner distance before you start the machine, takes 1 minute.