One more thing about seats:

Even without E+, on your outbound UA902 there are a bunch of aisle and window seats (with standard pitch but at least not a middle) available towards the back of the plane. Call up UA and ask for one. They really dropped you into the worst of the worst with 34E - a middle in the 5-seat center section with 2 people on each side of you. You can try to do a fake booking on united.com without logging in for a one-way for that date/time to be able to see the seat map, then ask them for what you want that's open. For example, rows 38-41 seats B (aisle on the left 2-seat side of the plane) or H (aisle on the right 2-seat side) are all open, along with a lot of the C (aisle on the left of the 5-seat center) and G (aisle on the right of the 5-seat section). Any of those are better than what you've got now.

On you return UA8827, which is really LH418, you do have an aisle seat. It's way at the back of the bus in 55C, 2nd to last row I believe, but I think it's also in the only 2-seat section of coach - further foward there are 3-seat sections on the sides and 4-seat middle section.

Anyhow if you put your United FF# in the reservation by calling united, you will then be able to log into united.com (or create a login account with your FF# if you didn't use united.com before). Go to "View Itineraries" after logging in and you can change your seat on the United flight yourself. You may or may not be able to change the seat via united.com for the UA/Lufthansa codeshare on the return - UA is flaky when it comes to accessing codeshare seats.

You won't be able to preassign seats for the intra-Europe flights. Typically most European airlines don't preassign seats for intra-European flights.