Let's see if I can help you out:
1) No, you do not have to flush/bleed the brakes when only changing rotors and brake pads. The brake lines are not disconnected from the rear caliper, so is not necessary unless you are not getting good pressure to the rear brakes.
2) Not sure why the person put a brake bleeder kit on the bleeder valve just to press in the caliper piston when you can just take off the brake master cylinder cap and monitor the fluid level. Maybe he was going to bleed the brakes anyway.
3) when you compress the rear brake caliper piston to install the new pads, you must have a special tool. Most auto part stores rent/loan this tool. On the rears only, the caliper piston must be turned while being compressed back into the caliper. You could try all day to compress it without this tool, and it won't press in. (Ask me how I know as I found this out doing my first rear brake job). If you go to a parts store and ask for the tool, they will know what you need.
4) The only thing you need to do with the master cylinder is monitor it while you press in the caliper pistons to make sure it doesn't overflow with brake fluid. I use an old turkey baster to suck out/remove the fluid if there is too much.
Hope this helps.
1996 Mustang GT Vert
'96 Mustang Coupe V-6
2004 Expedition 4x4 EB
14 Mustangs owned to date. Wish I still had them all!
"Regulators, mount up!"
"There's no replacement for displacement!"