The answer to your questions are:
1) No. Aelia Jusa is correct. Once you make your choice, the other options are "closed" (so to speak).
2) Here is the link to Baldurdash:
http://members.home.net/mrkevvy/tweaksandcheats.html
Just scroll down until you find the "Concurrent Romance" script. This allows you to have all 3 romances. I plan to do that with my next game. This time around, though, I'm romancing Jaheira. From what I hear, there are some additional quests available at the end of the game for following this "romance" thread.
Just below the "Concurrent Romance" script is an "All-Race Romance" script which allows any race to have a romance.