I'm an officer of the board in our mixed league and an active captain in 2 leagues and this is what I've known/seen...
How extensively does USBC track individual bowler statistics? Do they only track major achievements like 300's, 800's, and book averages? Or, do they track every score of every sanctioned game that you bowl?
I don't think they do individual stats. I know the secretary turns in the achievement score to USBC. I doubt USBC actually track individually scores. It's up to the individual. There's too much data to collect. It is a good question, which I'll send an email to USBC to find out.
I'm not sure how long USBC keeps the stats. I did look up a person on our league, who past away around 2004, and he still has his book average. I don't know if they do any type of purging of old data.
As far as leagues, I'm not sure what the league rule is, but I know that the secretary on our Men's league looks at the book average for the past 3 years. You may have to ask your secretary. We had a guy who hadn't bowled in over a year, and his book average was used from that last time it was recorded.
Regarding a tourny, I've gone to a few local ones and they go by, I think, 5 years.