The only way to loosen the belt is by moving the entire tail boom in, towards the body.  I wouldn't do that, because that's not necessary.  I don't really know how you took off your gear, but as far as I can imagine there's only one way to do it, so I assume that you can just stretch the belt just enough to get it off the old gear, and put the new one in.  After that's done, stretch the belt back over the gear.  As far as your throttle problem, I've experienced it too.  I still don't know why it happens, but as it turns out, the TX somehow lets "neutral" drift in the direction that you trimmed to before, so what I did to correct this ostensibly is that I reset my throttle and tail trims to zero before I plug in the esc battery.  It's scary, because intuition tells you that it would rev up and hit you in the face once you connect the battery, but don't worry it won't.  If it still kinda starts, you can just add a little bit of trim for that flight and remember to set the throttle and gyro trims to zero after you unplug the battery from the esc.