Many times paper is set behind plaster , sometimes 15 lb felt, this did not have asbestos,
if it is thicker ground & compressed paper that was used for a while in the 30’s and 40’s that stuff was easy to burn so it probably did not contain asbestos. To know for sure it would need to be tested, panel by encapsulated and then covered would make it safe if it was asbestos. I would treat any popcorn ceiling like asbestos as most did have it.
Mold goes away when the water goes away, so unless your toilet is leaking again, you have some other source of water or leak on the upper floor. This could be any number of things, even things not plumbing related, like water getting behind the siding, or a leaking roof with water running down the inside walls. As far as killing the mold that is there, I wouldn't waste my time. If you don't correct the water problem then the mold will return very shortly.
I need to figure out if there's still a leak or some kind of humidity problem. Is it safe to assume that because the problem got worse there is? Should I try to find the pipes and see if they're leaking?
Since you have had issues with the toilet in the past that would be the first area I would check. If the drain pipes for your upper floor are leaking then I would expect to see discoloration bleeding through the paint on the ceiling and the walls. If this turns up nothing then in the hole you have already cut, I would get a professional moisture detector device like the one a plumber would use.
These can help detect moisture and humidity in the air which can give you clues if you found the potential problem areas. Don't buy a cheap one because they don't work very well, I would instead see if a rental place will lend you a good one.
I think I'm seeing some black wood in there (need a better flashlight!).
This doesn't necessarily mean anything, it could be discoloration from previous water damage.
I would still like to hire someone to fix the plaster ceiling. Is it going to cost a huge amount extra if I cut through the mesh and they have to replace some of it? Does area make a massive difference in price for that kind of job? For what it's worth, I'm fine having drywall put up instead of plaster.
How big is the hole that you had cut? You can sometimes repair a hole by cutting a small piece of drywall and fitting that back in place, then using some thinset or spackle around the edges. Sand and repeat thinset or spackle two or three times until you have a smooth paintable surface.
Best Answer
If you had the ceiling removed by "professional certified removers" then you are free of asbestos. They should have sealed everything up, including vents, etc. before starting and all the particulates were pulled out of the air as the removal was done.
Now on the otherhand if you did it yourself or hired some shady under the counter removal company, your probably exposed already so nothing you do now will matter much.
There is a bit of hope in that the asbestos used was probably long grain(Old terminology, now they are classified by color.) rather than short grain. Long grain is less toxic than short grain but can still cause cancer, etc.
Cleaning everything, ducts, Furnace, filters, air conditioners, rugs, walls, floors, household items, etc., etc., etc. will help remove residual contamination, as will leaving all windows and doors open. Once spread all over your home asbestos is hard to impossible to remove completely.
Use masks if you do the cleaning yourselves and a water vacuum to clean the rugs.
It will cost a small fortune to have an asbestos removal company do a whole house clean. Also in many states doing asbestos removal yourself is illegal. So if you removed the ceiling yourselves don't get pro's in after the fact as its way too late and way too costly.