Please help with painting prep!

My grandmother lives in a 1940's NYC building, and my mom and I are repainting the inside. We scraped as much of the old peeling paint off as possible (50% probably) and had a guy come over who mixed Plaster of Paris with Joint Compound and put it in all the areas we had scraped. what we ended up with was a rough job but we figured we could sand it down and skim-coat it, and it would be fine. Then we found out that mixing the Plaster of Paris and the Joint Compound is not great, hired a professional plasterer for 2 days, who did this:

scraped off some of the plaster of paris/joint compound mixture
put on this pink stuff called plaster-weld where he had scraped
put EasySand 45 above the Plaster-Weld)
skim-coated twice over that with just Joint Compound, not mixed with anything

We sanded everything lightly after he left and everything seemed beautifully smooth, white and lovely. 3 days later, when we were wiping down the ceiling and walls to clean the dust off so we could apply Killz Oil Primer, we noticed about 7 spots in which the skim coat was cracking - when we touched it it felt totally hollow and big (about a foot) sections pulled away easily, unattached to anything.

We re-did the 3 steps in those places- plaster weld, easy sand 45, skim coat with joint compound - but new areas keep popping up, like Groundhog Day.

Should we just keep doing the process above and hope that it stops cracking, or is there another product we should use? We can't afford to hire someone to finish the whole thing and paint.

Any advice would be appreciated. Thank you!

TL;DR:
put Plaster-Weld, Easysand 45, and Joint Compound on some cracks in the ceiling. seems to work in most places, but its cracking now in various spots. is there another product we should use or should we just keep doing that and hope it doesn't crack again?

PHOTOS: http://s41.photobucket.com/user/plasterproblems/library/?sort=6&page=1

Hi welcome to forum. Im no expert but have done a lot of patching in my day. The first mix you made was much to hard to sand as I think you found. I would have just used joint compound from the beginning. Any real cracks where the plaster has cracked over the years will come back if you don't tape the crack and the mud over. I like the self stick mesh drywall tape. You do have to build it up just a little to hide the tape and feather it in over maybe 6 inches each side the tape. Mud has water in and when going onto old paint maybe is lifting the paint. Mud also shrinks and sometimes takes sanding and a second coat or more. I use kilz 2 primer and will paint the surface before filling even. Drywall mud sticks fine to it.

I'm going to be on and off today I'm doing the exact same job in fact to my 100 yr old stairway. Others will help also just give us some time. Weekends are busy for most. Good luck

I would just use joint compound. On the cracks it might have been wise to cut a v groove along the whole crack then tape with paper joint tape and joint compound then two coats of mud floating it out with a 12" mud knife.

Before you kill all your brain cells using Kilz Oil Based primer, consider Zinzer's odorless Oil base primer.