Put the Mirror on the mantle and the TV on the ceiling?
What TV?!?!
Ok, look. This is a building from 1892 with very ornate original molding. The walls are plaster on top of, well, different things in different places. In this particular case, the wall above this mantle is a section that comes out from the main wall by about 40cm and is where the no longer operational fireplace is. It seems to be brick (from the colour when I drilled) behind about 3/4 of an inch of plaster. It's a fireplace, there are no studs.
I drilled two holes where I wanted the mirror hung as I've had it in two other apartments in the last 13 years. I used brass picture nails made for 50lbs (the mirror is 32) that go in at 45 degrees with hooks underneath. You must know which ones I'm talking about. But as soon as I put the mirror up, the right one started causing the plaster under it to bubble up. About two inches in diameter. Like the weight of the the mirror was lifting up and out. Like digging a shovel into snow on an angle and then pushing down on the handle. Sorry, it's the only way I can think of that makes sense to describe it. It was clear it wasn't going to hold so I took it off.
The last place we had it up in, I had drilled holes into plaster that had concrete behind it and basically placed the nails in there. Even a bit loose, they worked for years. Before that it was, again, two holes drilled into old plaster and then brick. I simply don't get why this plaster is buckling like this.
Does that help? That's why I thought leaning it might work better. The mantle is marble and has a sheet of glass about 1/2 an inch thick along the top.