The phase wires can probably handle more than the motor windings, since they are bigger and have better cooling. The motor windings will probably fry first. It's still not a bad idea to oversize the phase wires to minimize losses, but you can't make them much larger where they go through the axle. Using wire with a higher temperature rating would be good through the axle.
A Sevcon controller will definitely be more robust than a Crystalyte. It's probably about 5 times more expensive too. If you can afford one, I don't think you'll be sorry.
Two controllers in parallel has been done before. It's real prone to blowing up though. If one switches a few nanoseconds before the other one, you can get shoot through (instant destruction). I think you'd have to tie the inputs to the gate drivers together to get the timing close enough.
"One test is worth a thousand opinions"