Skimming ceilings

It depends If you're ceilings have a stipple effect artex finish with quite shallow peaks they may just need a light scrape and then primed with blue grit before skimming. If you're ceilings have a fan like artex finish you will probably just need to prime them with blue grit before plastering. If you're ceilings are papered they will most definitely need scraping to remove the paper, if you leave any paper in place it will lose its adhesion when you skim over it and cause bubbling.

Yes if the artex is not a heavy pattern

Yes especially if the ceiling haven't been stippled/artex.

Mostly all artex with a spiked feel to it will need the first initial scrape! It all depends on the surface your working with if it’s artex u can pva and bonding/multifinish 20% bonding 80%plaster build it out, then second coat with multifinish leaving a flawless smooth finish However if the ceiling is papered you will have to remove ALL paper and use a sealer like blue grit or SBR and in most cases PVA will do the trick this will adhere to the plaster If the ceiling is cracked and in disrepair you also have the option of mesh coat And the option or re~boarding if really bad.