A crack is a sign the foundation is sinking. Some cracks formed when the house was built and settled and are fine.

Others are not fine.

It probably should. Guess it depends on how wide of a crack it is. I've repaired cracks up to ~6" though no problem. Might run into problems if it's wider than that as the patch can "bubble" outwards and become more noticeable. Higher chance of happening if it's an exterior wall that is prone to temperature changes than an interior wall. Can also apply in stages and let each small patch reduce the hole size and properly cure versus one big glob of compound trying to cure. If the visual doesn't bother, then should be ok as an interim fix i believe

Tape and compound will do it. Try to sand down to the bare sheets beforehand so your tape and compound dont stand out too much.

Use a long, flat sander to get the finish as smooth as possible, and then be sure to topcoat it with a finishing compound, it makes a big difference.

The cracks are where the sheets of plaster join and one of the sheets is out a few mil further than the other. Would you reckon whacking a few nails in it to get it flat first or would joining tape be enough

Yeah definitely, if its loose and you can move it in, definitely smack it down with a hammer and nails

Screws. For the love of God, spend the extra .0002 cents and use dry wall screws.

post a pic. Without seeing how the crack is formed you can't know if it is part of a bigger problem

I have been trying and this subreddit isn't allowing me. may need to get a link

https://imgur.com/a/n1K5Bj4 link to the cracking

It looks like it is bulging a little bit which can signify the load from above is too great and needs to be addressed.

I mentioned in my post we are going to replace the walls entirely. This is a shorter term fix so we can freshen the wall up

I use Atack fiberglass drywall repair tape.

Works for me, there are lots of options though.

Yes to screws.