It is a tight fit but the panel snaps back into the ring.
No. The flange is approx 14" x 14". Without an opening that large, it wont sit flush to the wall/ceiling.
You can insulate that area of the pipe if your really concerned. Domestic hot water should be fine, if it's heating hot water or steam piping, and the panel is ABOVE the pipe, it may not like that.
The biggest access panel we offer is a 14 x 29 - item #34044.
16inches by 16inches
I think you're asking if i person could fit through the 14" hole framed by the access panel. They'd have to be a VERY small person! I seriously doubt it. I wouldn't consider it a security risk. If you're asking whether you can reach into the opening and access plumbing, then absolutely - that's what it's designed for.
As the manufacturer (Oatey) said in their response, pry it open with a thin, wide flat blade screwdriver in one of the 8 small slots around the perimeter of the door. The door is a VERY tight fit in the frame. I chose to file down the "catches" until it was easier to snap in place. Just don't file too much!