So if I already did the weekly strike on the highest level and got the 9 coins, can I do it on the lowest level and still get 3 more coins?
Nope!
What is the design decision behind being able to only equip one exotic armor piece?
That's be OP as hell. It's designed so you build the rest of your legendary gear around the exotics. I couldn't even imagine running around in PvP with a Suros and Ice Breaker. That'd be fun though lol