As above, splice the paper.
Lining paper is a dream to work with (once you get used to it).
Image #2, if the wall is out square, no problem, just splice the paper.
In the above scenario, let's assume that you are working from left to right- as you get to the return, you may find that you cannot stretch the paper enough to butt both pieces up. Hang the paper, then hang another piece over the return and let it hang over the last bit that you applied.
Note: both bits of paper still need to be wet.
You will then be able to cut through both bits of paper and get a clean join. Do not do it free hand, the paper will (very probably) rip.
I use wide filling knives to cut against, my longest is about 12" but I use smaller ones as required. The straight edge holds both bits of paper in place as you run the knife down.
By way of an example, here is a
wide knife. I have never used the web site, I just wanted you see what I was talking about. The handle allows you apply sufficient pressure against the paper as you cut through, so that it doesn't rip.
I really recommend using a 9mm snap off blade knife. As you cut through the two layers of paper, you will hit the plaster, that will quickly blunt the cutting edge. You can snap little bits off as you are working.
My go to is
Olfa with the black blades. The Olfa blades are impressively thin, but generic blades will suffice.
Once you make you the cut line, remove the top part (excess), then peel back the top layer and remove the excess paper under it. You may, or may not need to brush in more wallpaper paste given that some of the paste will have soaked in to both the plaster and/or other layer of paper.
Important- wipe away all excess paste. The emulsion will reactive any old paste.
If you can see a slight ridge, use 180(?) grit sandpaper to lightly sand the lining paper (when dry).
If done properly you should not need to use any filler. In the event that you need filler, I would recommend either Red Devil OneTime or Toupret RedLite. Both are lightweight fillers with a very low water content. Both are unlikely to reactivate the glue under the lining paper. If you use a filler that you mix with water, the water content might activate the paste and lift the paper as the filler is setting. .
It is worth noting that both fillers are incredibly soft- softer than the lining paper. Ultimately, you are only, hopefully filling a 0.2mm gap
It might be prudent to put a coat of emulsion on any joins you want to fill though- it reduces the level of suction. And, if using 180+ grit, you can sand through the emulsion- kinda using it as a thin filler- that only applies to matt emulsion though- silk is a different proposition.
I hope the above makes sense, if not, just reply.
Best of luck
