Well, I'm not an expert on Beta etiquette by any means, but I do have a couple of thoughts....
1) I don't really think it would be "bad form," but it might be nice to at least meet them. I would go to both, but if you must choose and you are truly considering joining the other chapter, attending the Thursday meeting makes the most sense.
2) Attending the function will not obligate you to join the chapter. In fact, you could be really upfront with them about your idea for a Friendly Venture. They will be proud of you!
3) I think that sending a letter is a wonderful way to get in touch! Letters are such a pleasant surprise when your mailbox is usually bursting with bills and junk mail. Also, letters have a wonderful formality to them...kind of an old-fashioned appeal. I did the same thing when I contacted a local legacy. Unfortuanately, she had moved away, but her mother (also a Beta Sigma Phi) sent me a very nice e-mail offering her help and encouragement. You could always follow up with a call in a couple of weeks.
4) Sorry! No real advice here. I guess just be comfortable yet classy. The other guests will probably be "cleaned-up" as my mom would say
Good luck!
Erica