I guess you are asking for LH147 as Feeder.
It will arrive usually short to 1pm at FRA.
TG921 will leave from there at ~2:25pm
As long as LH147 has no delay and the Sec.-lines at C are not too crowded, you are safe.
But there is no guarantee! Especially the later can be a problem.
I would choose LH145 even if it is way too early
Yes. Thai-Member Staff usually will greet and lead you at FRA as soon as you step out of the Bus entering the building. They will hold up a Name Board. Just follow the Transit. They will guide you (walking) up to, usually, C-Gates.
When you return to FRA, Thai-Member Staff will greet and lead you right when you leave the Plane and guide you through Immigration (Diplomatic Counter) and Security as well all the way up to (if you flight goes back to NUE) Sen-Lounge at A26. (walking again) ^^.