I did backpacking Rwanda and Uganda, here's my tips:
For the gorilla trekking the only thing you need is a car to take you from Ruhengeri to Kingi - and onward.
What I did was that I decided to stay in the more expensive hotel there, and then I asked if there was a car going tomorrow, there was! and I got to join their car - and then shared the cost of it roughly 20USD each.
If you are in Ruhrengeri the day before, I don't think that should be to difficult to arrange, as 48 tourist trek each day, if you stay in Kingi i doubt that you need a car at all)
After trekking I aslo got to learn that there is a local bus going from Ruhengeri to Kingi (that makes sense? All the guides and park rangers need to get there in the morning right?)
Then, I would suggest to take the long distancebus Kigali-Kampala (10-12 h) (or a flight? - but be aware of departure taxes) And in Kampala stay with Kampala backpackers (300USD) or the Red Hot Chili Hostel (240USD). They both arrange safari to Murchinson Falls, about the same value for what you get, but with Kampala Backpackers you also get the option to do Chimp-trekking.
Make sure you also join a day drip with Nile Rafting (100USD, you'll get transport to and from Kampala, rafting, breakfast, lunch and dinner + drinks)