Jpkempplasterers

If join wall to a cieling all way tape any joins but only after you floated the walls and allowed to dry

The easier solution is to try blue grit and allow 24 hours to dry then skim.if this dont work use course sand papper to score the paint ..if not desired then remove old plastering

Easifil for small area is like polyfiller but nothing can replace the strength off a pure two coat skim

Plasterboard is like a sponge as most off modern plastering materials and all gaps between walls and floors should have a gap so the air flow continues and drys the wall if gaps are to big there many alternatives to packing it out first replace size skirting board if available

A water prove cement mix on both then skim both after a curing period off seven day so you can tape any shrinkage or crack along both joints