Rimma was a senior guide and did not direct all the tours. They did and do have other guides. She is just the one that got the most publicity. BTW, not all the guides are associated with Chernobyl InterInform, but all requests for entry to the Zone goes through them.
The rule of thumb in the Zone is that you can work in the Zone for 2 weeks and then must spend the next 2 weeks out of the Zone, so there are different guides at different times.
Only people with a lot of experience in the Zone are allowed to travel freely, though you still need permission for access. Virtually everyone is required to have a guide with them.
I was able to take my time, but rushed myself so I could see as much as possible.

I think timing depends on your arrangements and who else is with you (e.g. if you go through a tour operator there is probably very little leeway).
Due to the people I know over there that helped arrange my trip (I went with friends, not a tour group), I got more of a personalized tour and got to do some things that most people can't do (e.g. see inside the Reactor 4 control room, thoroughly investigate some of the villages). I also got to see more because I was there for 2 days.
I think on most any tour, there are certain things they want to show you and there is only so much time in the day. My guess is that they will be patient with you and eliminate some things from the tour depending on the time left. Trust me though, there are not many places that you would really need to stand around for a while to get proper light, and inside the buildings, you don't have much of a choice (my SB-800 was great indoors, and the built-in flash worked fine too in some cases).