Don't have answers to all your questions but getting a taxi in the streets is pretty easy in most places.

Walking around wouldn't really be recommended, the things to see are usually pretty far apart and it's not a city like NYC.

If you can find a tour, that would work well or just pick a few places you'd like to see and get taxis to each. Definitely go to the corniche (the seaside), get a strong coffee or Moroccan tea and smoke some sheesha, it's the best in Bahrain (Bahraini apple flavor).