Hope you have fun!  I'm relatively new to the hobby myself (only four experiences in the last year), so take this response with the grains of salt it deserves:

1)  Yeah, that's pretty standard for screening.  If you sign up with RS2K, that's the info they tell you to give providers.

2)  Providers sometimes drift in and out of the business, so it would not be altogether surprising if there was a gap.  If there's relative continuity before and after the gap, the most likely reason is that she took some time off for personal reasons or otherwise limited her clients for a period.

3)  All providers will tell you that tipping is optional.  I personally believe in tipping - these women are performing quite the service for you, and they have to eat, pay rent, etc.  My tips tend to be more generous for agency girls because agency girls have to split the fee with the agency.  But I do strongly believe in tipping.  Others may have a different philosophy.

How much?  Well, that's an entirely separate question, subject to a variety of schools of thought.

4)  You don't know for sure if the provider is LE, but reviews here are a pretty good gauge.  Someone with 6 pages of reviews (particularly positive or glowing reviews) is far less likely to be LE than someone with few reviews.  That being said, if LE really wanted to get aggressive about cracking down, they'd look here.  My sense is that LE tends to focus its attention less on the kinds of women who are reviewed well here and the men who find them here, and more on things that have some broader community impact, like streetwalkers, drugs, etc.  It seems to me that incall may present more of a risk because of the possibility that the hotel management will complain to LE if they suspect a provider is using their facilities.  Others reading this may have a different and more informed perspective on the issue.