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Option 2 sounds interesting. So sand it flat, apply lining paper and paint? I have no experience with lining paper, can a noob do it?
Hovis-
Yes a noob can do it. The type I'm using at the moment is expensive but I've never found it easier. I have a big tub of ready mixed paste which makes life a bit easier but you probably won't want to fork out for one of those just to do a wall or even one room.
Erfurt variovlies is the wallpaper, it doesn't stretch or shrink and the finish is like cartridge paper so you get a great paint finish. (decoratingdirect.co.uk) Advantages of using this are you don't have to soak the paper to stop it shrinking on the wall.
If you do go with packet wallpaper paste mix it exactly by the instructions and make sure you give it enough time to get rid of lumps.
Make sure you prime the walls with something, I use Gardz but you can use wallpaper paste or PVA, this will stop the filler from soaking up all the paste.
Then paste the wall with a roller or big brush, don't paste the whole wall just work ahead of yourself one sheet at a time.
Measure the height of the wall and cut a piece of lining paper about 100mm longer, measure twice, cut once! Place the wallpaper on the wall and smooth out with Zinsser walwiz or a paperhangers brush.
Have a very sharp knife, Olfa do a specialised one with a thin blade, the blades get blunt very quickly and you won't be able to trim the paper properly if they are not razor sharp. You can use the smoother to get a clean cut.
While it drys you might get bubbles, don't play with them they will most likely disappear when it drys. It's very common to have loose edges where the glue gets squeezed out or is dry because of handling the paper onto the wall. Fix those seams with a decorators syringe full of wallpaper paste.
If you do get set up with paper and paste and a few tools to do this job you'll probably end up wanting to do it everywhere.
One other big consideration is keeping everything clean when you are doing this. You need the walls to be clean of any bits so sand lightly and check (some people do this with by running a hand covered with stocking over the wall). Make sure the floor is covered and the covers are clean. Any little bits that get on the back of the wallpaper or in the paste will stand out, you can not get rid of them once the wallpaper is up.
Most people get caught out coping with corners, lightswitches, plug sockets etc. There are plenty of tutorials but I always remove the sockets and switched from the wall and bag them in plastic bags. Do be cautious if you go this route. Coping with switches and sockets is the toughest part of the job to get right. If you find yourself messing about you can always add more glue but be careful about small pieces of plaster getting in to the glue from the edge of the sockets!
https://www.decoratingdirect.co.uk/viewprod/z/ZINWALW/
https://www.decoratingdirect.co.uk/viewprod/m/MAVNWLPV/
https://www.decoratingdirect.co.uk/viewprod/o/OLFA_12-5MM_WALLPAPER_CUTTING_KNIFE/
Airhead
Those types are a bitch. You're right to wonder how the board was fixed, in my experience it needs to have something solid bridging the join on the back side of the plasterboard to even have a chance of not cracking.
Since you don't want to have to remove it to find out I'd say the best way to try and fix it is with Toupret fine scrim tape and toupret fibacryl. Dig out the crack a to a v shape and apply fibacryl then scrim tape a bit more fibacryl and then feather it out with tx110.
Or scrub all that and line the wall, I would do that if I had to guarantee it didn't come back.