I actually have a similar UA/LH codeshare itinerary in a few days, except through FRA. I will answer what I can, but am interested in how others answer your questions too.

1. In the past I've able to check in once with UA. But I can't recall how that would work for a seat assignment. Hoping someone on the forum can answer that part.

2. Yes, if you are in coach. With LH, you can't select a seat until check in. My experience has been they only allow it if you are in business.

3. Not sure, I don't check a bag.

4. GE gets you nothing until you come back to the US.

5. Yes, you can use a LH lounge. Use the lounge finder on the Star Alliance web site to see which you're eligible for. The Senator's lounge is nice, but I'm not sure just being *G gets you in that one. The lounge closest to your gate is probably best. Your 2 hours at MUC could go pretty fast actually.

6. I've only been through MUC once or twice. I don't remember it being difficult to transfer at all. However, I do recall the plane parking at a remote location, and boarding a shuttle bus to the terminal, which takes a little extra time. But you have a long enough connection.

Hopefully others will add to my responses!