Maybe a dumb question, but i have not been able to log into Remix yet to test but:
Can you transmog your timerunner to wear your retail gear? Or can your timerunner only transmog what’s been discovered in Remix?
This greatly affects what class I’d make 
Thank you!
You could in Mist Remix so don’t see why you couldn’t here. You’ll need to move a character to Retail to use anything new you earn in remix on retail.
Can anyone who has a made a legion timerunner confirm this?
Depends on the transmog, I’m sure the level requirement still applies to things like war within transmogs.
I can transmog my legion remix priest to use a lot of items, but I can’t transmog the wooden sword dagger / offand from War Within’s childrens week quest.
You can tmog your retail gear. My dh is rocking his elite pvp set
Yes, just confirming above.
Though, outside the weapon, might want to hold off for a while. You get upgrades every minute!
Yes. My guy is wearing my horsemen duds currently. A lot of set items via raids, etc have minimum level requirements. They will become available as you level. At least that is how it works in the normal game, so I assume it carries over.