Main hand/off hand tool differences?

[Orophin]

Nope. I noticed in phase 3 that some recipes automatically made me use my off-hand tool as a weaver. Not sure if it changed in phase 4.

The item you're crafting determines which tool is displayed. I'm pretty sure that's just an aesthetic thing. It wouldn't make sense to spin thread with a needle or craft a tunic with a spinning wheel.

Also, if you un-equip your offhand it'll of course default every synth to your main hand even if it makes more sense that it'd use your offhand.

The stats add together for your total control/craftsmanship now. If you don't use offhand now you will be incredibly gimp.

This. The stats are additive now and as far as anyone knows one tool doesn't do anything different than the other functionally.