Cutting porclein tile

Recently I have moved a house that has old bathroom maintained poorly and used heavily. There was a lot of mildew on walls/ceiling in it. So I started to fix it. First I re-painted walls and ceiling with mildew-proof paint after some preparation including removing the mould. And decided to remove silicon caulking around bathtub and some on grout as they looked ugly with mildew. But when I removed the silicon, some ceramic tiles on the wall near bathtub fell off. My original plan was just to re-apply silicon and maybe some grout around bathtub but ... I am not familiar with tiling. What do I need to do in order to adhere the tiles back to wall? What kind of adhesive is recommended? Do I need to take off some other tiles around the hole and re-stick them? Please help!

Most of my new/old house has ceramic tile on concrete and there are several places that have ‘hollow’ spots. I am guessing that the previous owner/installer did not use enough thinset in these areas. There is a creaking sound whenever I walk over some of these spots. So I have a couple questions. Short of taking out the tile, is there any preventive measure that I can do to stop the tile from eventually cracking and popping out? The carpet in a bedroom has been removed and I can see the thinset for the tiles in the doorway to the hallway. Two of them sound hollow and I can see that the thinset does not reach the tile. Is there a way to add thinset (or something else) into the grooves of the existing thinset to reinforce the tile? Thanks for any help.