Will I break my Dahon Mu Uno?

Weight limits are usually based on what the wheels can take rather than the frame. Also like with most things nowadays the manufacturers build in a margin of error so you can likely ride it safely 10-15 lbs above the actual limit.

Riding on compact gravel should not bother the bike so long as you keep it clean - make sure dust isn't getting into the hinges. To rule out any issues with the hinges clean them thoroughly and apply a light coat of quality lube something like Dumonde or Finishline. Then ride the bike and see if the creak gets any better. If it does, there may be some play in your hinges. A good bike shop should be able to adjust that play for you. If it does not get better the culprit is likely somewhere else. Possible sources could be the saddle mount or bottom bracket area.

Depends on the frame material. I can speak only about aluminum.

A cracked aluminum tube produces a weird type of squeak - you recognize it once your break you first frame. Before that, just inspect your bike for cracks at the welds.

Another thing is how you ride. Fellow 60kg MTB'ers claim they can break a DH bike in 10 drops to flat from 1 meter if they want to. I don't know if that's true, but am sure MTB skills can help safely ride a road bike through off-road terrain.

Keeping your ass off the saddle, keeping knees and elbows properly preloaded but relaxed, reacting to impacts progressively (slight impact - be limp; harsh impact - push back exactly as much as to exhaust your whole preload), line selection, hopping over sharp edges etc.