I've built this machine 100 different times in my head!!!

[JAZZCNC]

#1: This guy is ok. Most of them are the same spindle and probably out the same factory.?
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#2: Honestly there is no solution to match driving from both sides whether twin ballscrews, R&P or belt drive it's by far the best and easiest way to get accurate stiff machine. The wire/drum is just a bodge and best doing it correctly first time.?. . You'll only end back there in the end.!!
Unless your machine is narrow format, which it doesn't look, then single screw just won't work good. Making gantry wide helps but there's a limit before it gets unusable or feasible.

#3 Can't answer that question without knowing which make and what size linear rails. What I can tell you is even the lower quality units are capable of handling forces far beyond what you throw at them or what your machine design will handle, so it will fail long before they do.!!

#4: No contest here linear rails win hands down in every department by a very big margin. They can't be compared really in accuracy and performance terms.
The other advantages are longevity, they will easily last several years longer than supported rails, even in very harsh conditions. Hassle free usage, fit grease occasionally and forget.
Disadvantages over linear rails is they don't tolerate poor workmanship and misalignment.

#5: Show us and will comment but until then not sure what you mean.?

Now your design.?? . . . You have a problem Houston (several actually).!!! . . . . but this one Is a common mistake easily missed.!
The linear bearings on the Y axis can't possibly be fastened like you have them drawn.? Chicken and egg problem.!! Fasten plate on one set and you can't access the bolts for the other bearings.!!

I'd also consider using different thickness material for the Z axis.? Those thin plates, even with the bracing will resonate while cutting which will affect quality of finish. Don't rely to heavily on the Cad simulation, without inputting all the variables and cutting forces then it won't show this. I know from experience that if you plan on cutting hard woods or hard material like aluminium then your Z axis just isn't strong enough.
Just remember no matter how well built or how strong or accurate the rest of the machine is it's the Z axis that takes all the cutting forces
and if this flex's or resonates the rest don't matter a jot.!! . . . The machine is only has good has it's weakest link, this Z design is weak.!
( Also Don't rely on the Linear rail adding strength they don't really add much.!)