Advance seat assignment on Europeans flights is possibly only to either:
1) HONs and SENs; or
2) those flying in business class; or
3) those who have purchased a full flex Y/B fares.

There is no E+ on LH. Business and economy seats are identical; the only difference between between M and C is that in C the middle seat is blocked and is converted into a table. Oh, and of course, the curtain. You get get your seat while doing an online check-in (23 hrs before departure at lh.com or 24 hrs before at aua.com). Emergency exit can't be assigned during online check in.

As for the luggage... Hmm, that's a tricky one. Are your FRA-LAS flights on one ticket? If not, agents in FRA may argue that for your FRA-BRU you can only have 'weight *G allowance.' If the complete trip is on one ticket, and your EU-US flight is in C, I'd imagine you can have 3 pieces 32kg each.

How did it happen that FRA-BRU is in M?