As RC and JVN have said before, the idea of the VersaChassis is to be able to build a great drivetrain with only a hand drill and a hack saw. Everything is pretty much non-critical toleranced (ie, clearance) or match drilled.
Yes, that is correct. I believe a 1" hole would work well, as you want clearance for your hex, plus some for tensioning. They sell you extrusion with no wheel holes drilled, you have to do that yourself (you can also just buy regular 2x1 tubing).
One wheel (usually the center) is direct driven out of the gearbox. The outer wheels are either chained or belted to the output shaft of the motor, where the sprockets or pulleys usually live inside the gearbox (but outside of the tubing).
The output shaft goes through the bottom of the gearbox, through the frame tubing, and into the center wheel. This way, if a chain/belt run breaks/skips, your drivetrain can still drive.
4)Rivets are really better than bolts here (lighter, quicker to install, etc), but you can use bolts too. They’d need to go all the way through the tubing though. As you said, putting a nut inside the tubing would be unrealistic.
Here’s what you do to put together a VersaChassis: Figure out your ratios, wheels, and frame dimensions. Order parts. Then, take a hack saw (or band saw) and cut your frame 2x1 to length, trying to keep the cuts relatively square and accurate piece to piece (+/- 1/16 would probably be fine, +/- 1/4 likely would be pushing it). Figure out where you want your wheels to go in the tubing, and drill 1" clearance holes there. Press bearings into your versablocks, and clamp them on the tubing in the approximately right locations. Make sure that the versablocks are assembled such that they’re with the slot up on the outside and the slot down for the inside wheels.
Then, figure out about how long you want your wheel shafts to be, and cut them to length (with a hack saw is fine). You’ll use shaft collars to retain stuff on the shafts. Slip them into place in the versablocks, and add on the necessary wheels, sprockets and hardware. Either test fit your belts or measure and cut your chain to the right length at this point. Then, assemble your drive gearboxes, and mount them to your side frame tubing by bolting them onto a half versablock located on the outside of the tubing, with their output shaft going through a bearing in the versablock. You might have to do some match drilling at this stage. Once you’ve verified that your wheels are about where you want them, drill a #7 hole around the midline of the tubing close to the outside versablocks to mount your cams. These mount with a 10-32 button head, with the head on the outside and the cam on the inside. To tension each wheel in turn, rotate the cam with a wrench until the chain is tensioned, then tighten down the versablocks and the cam screw. Test your drive sides before assembling the frame to make sure everything is running right.
After you’ve gotten your two halves of the drive assembled and you can see that they’re working correctly, it’s time to assemble the frame. Take the two sides of your drive, and your front and back tubing, and arrange them in the shape of your frame (sort of a fat H on it’s side, like in the picture you linked to). Then, take the Vex Pro T shape gussets, and clamp them onto your frame, so your frame is essentially assembled how you want it, just with clamps holding it together and without any rivets. Then, match drill each hole in the gussets with a #10 drill bit and install 3/16" diameter by 1/4" grip rivets (or any other size rivet or screw you like) in each hole. Once the drive is assembled, test driving it around (with an old electronics board or such) to make sure it’s preforming as expected. Try making it turn on carpet, as this can be a huge problem for robot drives. Finally, when you’re satisfied that it’s strong and working as expected, cut a wood bellypan to size, and rivet or screw it onto the bottom of the frame. You’ll likely want clearance holes in it for your gearboxes.