Generally, the drain is in two (or more) parts. The part that goes in from above (silver or whatever) uses plumber's putty to seal it. You put a small 'rope' of putty shaped like a doughnut underneath this part, between it and the sink, around the hole. You then press the part down into the sink by hand and wipe away the excess. From below, you place a rubber washer, usually conical or pyramidal in shape, around the drain tube and press it upwards against the bottom of the sink. The cone-shape faces upwards, the flat side down. This is pressed and held into place by a large flat washer and a hand-tightened nut. Tightening compresses the rubber washer against the bottom of the sink and the edges of the drain tube. It also further compresses the putty mentioned above. Under this you usually have the tailpiece that goes into the trap. Since all of this is at zero-pressure, you don't need teflon tape or anything else. Just a good snug fit. The seal comes from the plumbers putty from above and the rubber from below.

"At least if I could find the leak, I could point them to the right place. Does anyone have any experience with brand names of snake cameras are good and what to look for?" Unless you have a very large hole or a pipe completely disconnected a drainline camera is not going to show you very much. The insides of pipes are not very clean or smooth (especially if they have been in service) and a small leak or cracked is not going to be visible. You should count yourself lucky none of the plumbers have wanted to take your money for a waste of time. Even with a camera, it can be hard to figure out where the camera is in the pipes after a few turns. Main lines tend to be pretty straight form the street to the house, but under-slab drain lines can have enough bends to make tracking the camera very hard. With plastic lines you can use a camera detector to locate the camera in the pipe (they give off enough electrical noise from scanning their image device) bit in cast iron lines (and then under concrete) the signals are very weak. The detectors are sold to allow a stuck camera to be found and removed.

Because it seemed important to know how bad the situation is, I removed the tiles and mudboard in the problem area. Underneath is just the sheathing. You can see that most of the wood is still in tact, but it's definitely rotted in the corner by the tub. That spot is still very wet, smells earthy, and I can dig into it with my finger. In a 4" x 4" area, I could make a little 1/2 inch hole easily. Now I'm wondering if the best solution might be to: 1) wait for wood sheathing to dry; 2) put down some kind of underlayer or mortar, and then 3) just put on some of that Flex Seal - as a temporary solution. Or, should I put down some kind of PVC board, or something like that over the sheathing - and then put mortar on that - just in case there's reason to worry about the sheathing in that corner giving in. and the PVC board would keep us from falling through the ceiling (until we do the full renovation)! Thanks everyone for your help!

I think the problem is that those boot things etc don't have enough wide opening to fit the middle part of this installation. The opening seems to be 1.5 inches but then the fittings after are 2 inches, then the final part is 3 inches (which usually is covered by the plate). The other side of the wall is the kitchen which is now finished and all cabinets are up. On the plumbing forum, there was a lot of advice (but hard for me to understand) but one person said this "He used the WRONG fittings at the wall so they project out too far. Have him come back and redo it properly. He is not a very competent plumber if he doesn't know which fittings he needs." Which to me seems like he used what he had in his truck instead of what would work for this particular job. I will send the picture to a plumbing supply house if this stalls to nothing and I'm sure I can order in right fittings and have them redo it for me.