Along with what Tristan said, the frame looks fairly light. I wouldn't be surprised if the frame flexed enough to make do without the drive module pivots. Though, knowing your design skills, I'm assuming you'll have more structure going on top to stiffen the whole assembly, hence, making my point worthless.
Do you plan on rivetting/welding/bolting the frame?
However, I'm at a loss as to how the tensioning slots work. Since the idler moves with the gearbox module, it seems like all you're doing is changing the tangent point of the chain with the wheel. I would expect the idler to move with respect to the gearbox, or to just use an eccentric idler/guide. That could greatly simplify the entire assembly.
Overall it looks very nice and clean. One of the better Mecanum setups I've seen.
BEN