1. You can delete bobs freely to keep things fresh if you desire. It offers no benefit.
2. You should NOT retire your ORIGINAL bob as he will have the highest experience bar. There is a trophy for taking a bob to the "peak of his career" which is attained once you fill up the experience bar (its labeled something different outside the US...)
3. I believe the experience is accrued through game days, and not races driven, as my bob with double the races driven still has a lower exp than my original bob.
4. Higher level bobs won't gather more money in racing... a bloke seems to have sorted out the payout policy just now. Here:
https://www.gtplanet.net/forum/showthread.php?t=186400