Connecting Thunderbolt 2 Mac to Thunderbolt 3 Display, what is the maximum resolution@100hz , will USB 3.0 work?
Yes in general. I use the same adapter to run all manner of USB-C and Thunderbolt 3 displays from 2013 Mac Pro and MacBook Air and Pro from the same era. You don’t get any extra resolution than the Mac could drive originally using adapters that shipped when the Mac in question was brand new.
I’m delighted with 5k displays running over the Thunderbolt 2 and all USB and Thunderbolt hubs I have tested work at full Thunderbolt 2 speed. Since USB-C and 3.0 is less speed than Thunderbolt - they all work fine in my experience.