A & B refer to parts of the terminal. AFAIK your LH flights will be in B. There are LH SEN and BUS lounges in the central part of B (large) as well as down the pier (smaller).

Showers need to be booked and there's always been a horrendous queue whenever I've tried to use them, book as soon as you enter the lounge (at the entrance to the toilets).

Yes lounge has smoking and non-smoking areas, however lounge is open plan so if dont want smoke sit well away from the smoking area.