Since I bowl in the ABT Chicagoland North Branch, I cannot speak for the Bay Division, but I can give you some insight into the ABT in general.
Most of your questions have been answered by other posters. I'd like to add that there is no commitment, whatsoever. You may bowl as often or as seldom as you wish. However, there are a couple things to take into consideration. As mentioned by another poster, there are events for which you need a certain number of tournaments or entries to qualify. Our big one, the Chicagoland Open, is coming up over Memorial Day weekend. In order to be eligible to enter, a bowler must have four entries between the first of the year and the beginning of the Open. The top prize is $10,000 in each division (we still run both Masters and Classic divisions). Last year's event attracted a total of 1562 entries for both divisions combined.
The other thing you should keep in mind is that if you go more than 120 days without bowling an ABT tournament, you will be subject to a rerate. Here's what that could mean to you: You say that you are bowling about twenty-plus pins higher than your last league average. You are right in saying that this is a good time to join an organization such the ABT. Your entering tournament average would probably be set at your highest league average over the past five years. However, once you have joined ABT, your tournament average is based on your ABT scores from that point forward. Let's say you come in at 148, as you mentioned. Let's assume that you don't come in and win a tourmanent right off the bat, but you continue to average around 170, as you say you have lately. You enter tournaments, cashing somewhat regularly, and maybe making a final here and there. Your tournament average would probably move up into the 150's, if you continued to do well.
If you were to go more than 120 days without bowling a tournament, you would be subject to a rerate. Let's say it's now next season, and you are now averaging 170 in league. Your ABT average would be adjusted upward, most likely splitting the difference between your previous ABT average and your new league average, putting you into the low 160's. If you were to let your membership lapse (you don't pay your annual membership fee) and rejoin, you would be adjusted to your new highest league average, 170 in this case.
I hope all of that was reasonably clear.
Anyway, I just want to say that if the Bay Division is anything like Chicagoland, it's great fun, and the people are really nice. I've been in ABT for nearly nine years, and I have formed many lasting friendships from it.