Yes. It is slower because the paint seals the paper against steam, but it can be removed and the wall made good ready for redecoration.
Tell us the pattern repeat and we will calculate quantities including wastage. Ordering all rolls from one batch matters, as shade varies between batches.
Almost always. Filling, sanding and sizing is what stops seams opening later. We include preparation in the estimate.
Yes. Both need careful handling and planning, and we set out the drop sequence before anything goes on the wall.
Usually, but it depends on the paint and its condition. A sound, matt, well-adhered painted wall is a reasonable surface once it has been filled where needed, sanded and sized. A heavy gloss is not, without preparation, because there is very little for the adhesive to key into. Anything chalky, powdery or flaking is not a sound surface at all until that is dealt with. It is decided by looking at the wall rather than by a general rule, which is part of what the estimate visit is for.
Removing it is almost always the better answer. Hanging over existing paper means the new adhesive can soften the old one underneath, and any seam or flaw in the layer below will eventually read through the new face. Old coverings are stripped as part of the job — including ones that have been painted over, which take longer because the paint seals the paper against steam — and the old adhesive residue is removed too, because it goes soft again when fresh paste is applied.
The first length is hung to a plumb line rather than to the corner, because corners in real rooms are rarely truly vertical and hanging to one passes that error into every drop after it. Paper is not taken around a corner in a single piece: a small amount is turned onto the return and the next length is hung to a fresh plumb line. Where a pattern runs right around a room the last drop will meet the first with a mismatch — that is geometry rather than a fault, and the job is to place it somewhere it will not be seen, such as behind a door or above an opening.
Most often because of the wall rather than the paper. An unsized or very absorbent surface pulls the water out of the adhesive before it has bonded, and something that was not properly sound underneath cannot hold the tension paper puts on it as it dries. Edges can also lift at the highest-stress points — external corners, reveals, above a door — or where a drop was stretched slightly during hanging. A small lifting edge can often be repaired; a seam opening because of the surface underneath needs the surface dealt with first, or it will do the same again.
It depends on what the wall is for. Paper is worth it where pattern, depth or texture is the point, or where one wall is meant to carry the room. Paint is the better answer for a straightforward colour change, for surfaces that get knocked and need touching in, and where you may want to change the decoration again before long, since repainting is far easier than stripping. Neither covers up a wall in poor condition — if anything paper shows more, not less — so the preparation question is the same either way.