sfsuperstar: "My husband's really doesn't like the ridge, and the rest of the ceiling is so smooth he thinks trying to blend a 3/4" deep ridge will look obvious, so his tentative plan as of this evening is to remove the ridge of mesh to try to flatten it, then try to redo the crack repair smooth." That sounds reasonable. You may have to widen the crack just a bit more and clean it out, trying to cut under the edges as much as you can so the patching material has a place to grab in to and hang on. sfsuperstar: "It seems that the sagging part isn't an edge of the mesh, but is a center part of the mesh -- like an upside-down mini-mountain ridge. Should he try to cut out just the ridge part of the mesh with an exacto knife, or does he have to remove the whole mesh tape (even the part that's not sagging)?' I would guess it depends on how easily it cuts. I've not used it before, so I can't say. If it ravels and gets frayed and stringy, then it may be hard to cover over again with whatever you use. sfsuperstar: "And should he re-repair the crack / removal damage with flat drywall tape and joint compound? Or is there something plaster-specific that he should use? I'm not familiar with the patching plaster you mentioned." It may be OK to use the drywall tape, but I would wonder how much it would give or move under changes in temp and humidity in the ceiling without breaking loose at some point. I really don't know! We got the patching plaster at the hardware store; it's dry and comes in fairly small bags, and you just add it to a small amount of water in a container and let it absorb the water, stirring or 'folding' the mixture only very gently. If memory serves, drywall compound is still water soluble after it dries, while the patching plaster is a bit more impervious to water once it dries. Drywall compound also has a tendency to crack easily as it dries if it's applied to thickly, and the patching plaster doesn't do that as readily.

I would do both. Sand out any ridges and then float compound over the whole area.

I agree that it could be settling, but those straight cracks also look a lot to me like a sheetrock joint where no tape was used over the joint. As for your first question about doing it yourself, it's possible, but it's not totally simple. You can't just use spackle on the crack, because it will open up again. The "easiest" fix would be to sand down a wide strip on both sides of the wall cracks, and then to embed tape, let it dry, and do a second or third coat to feather the joint out. Then you would have to re-paint with something that gave a heavy texture to try to make it all blend in. If you've never done work with joint compound, it's a skill that takes some practice and it would be easy for you to end up with the crack gone, but a more noticeable mess if you're not skilled with this. As for the ceiling, that's a tougher one because of the heavier texturing, and I can't tell you a good way to make this go away without a lot of work of the same kind. The only other think I can suggest is that there used to be a product you could buy in a spray can that laid down a thin, flexible film that bridged the crack and then you painted over it. It might not work well for you given the size of your cracks. You could try spackling the cracks, which would close them up better, and then try the spray can approach. No matter what you do, though, you're going to have to re-paint.

Lei Wei- I'm presuming that you are asking because you perhaps haven't made a repair like this before and/or don't know hard this is. It's not hard and the repair is easy, but it could be difficult to get a satisfactory result the first time you try this. I've done repairs like this for un-handy friends and neighbors a number of times because I've done quite a bit of sheetrock over the years and it is pretty easy for me. So I suggest you ask around to see if any of your friends have done this before, and could help you. If you have to hire someone, it might be hard to find a good person because the job is so small that many handymen would not find it worth their time. And, if you hire someone, I could see them charging a couple of hundred dollars because, even though it is easy, it requires patching it, letting the patch dry, coming back to level out the patch, letting it dry, coming back for possibly a third application, and then, finally sanding it and painting it to match. It's the multiple visits, not the difficulty you would be paying for.