1) There have been a number of times that I have stayed in the RCC past the elite boarding time, but I will still use the red carpet. Even if 6-7 people are in line, I earned the privilege to use the carpet, so what the heck...
2) If nobody is in line, I might use the economy lane, but it depends on the environment at the gate...
3) I have never told a GA that a general member was trying to use the RC lane, but I have seen people turned away a number of times. I flew from ORD-DEN on a 747 in August in C, and while boarding on the red carpet, a premier exec member tried to board. The gate agent turned her away...
4) I had this happen one time, I was traveling on a personal trip, so I wasn't in business attire while flying first. It did take the GA a moment of two to acknowledge me presence, but as soon as he saw the 1K and First Class on the boarding pass, he stopped the general lane and let me board... I got a lot of dirty looks...
But overall, I will always use the Red Carpet...