besunman

1. If you will be flying in from Paris and you are able to have your bags transferred to UA without collecting them this is a reasonable option.
You will have to do a walk from the A terminal to the B terminal which can take 10min (some people complain about this) and you will have to go trough security which will be probably very busy as July is the holiday month in Belgium. And afterwards you will need to present yourself at a UA counter for an additional check (do this before you go to the lounge, which is located on 1 floor higher, when you pass security 100m further there is a white stair this will guide you to the lounge).

2. If you take the train and just spend the night at the hotel near the train station I would opt to take the same evening a train to the airport and stay at a hotel near the airport. (as mentioned before the train station is not a very good neighberhood and certainly not if you have a obvious laptob bag with you).

3. As for the UA counter, you will have access to the priority check in lane. I think check-in starts around 7 AM Securty fast lane is for first, business class and GS members only. Boarding is done strictly and timely so 50min in advance of departure time.

All in all BRU is a relaxed airport, but as you are traveling on a holiday month expect that economy class of your plane will be full (C and F probably empty) and that security lanes may be longer than usual (also do not forget in BRU all international traffic leaver between 9AM and 12PM).

If you need additional info let me know