Roland, I rode half the Michelson in early fall, north-south, after a rain. No problem at all, other than wet grit clinging to the bicycle. Link to CG article here. Link to good map here. Link to excellent bike shop in Hill City.

The major exit for me was at Crazy Horse Memorial. Strangley, that is not an official exit, but it's an easy push up the embankment to the entry. Several service exits, least on the top half, and water/rest areas. There are a number of gates, none of which will be locked, in spit of how they appear(long story.)

No solid answers to your other questions.