you could just use a stain varnish as the different woodchips soak in differently and makes a good look, bit like rag rollering. try the look on a spare peace. or try a thin miss coat of emulsion colour to suit.
it could be the filler cracking if so rubdown and refill, prime then paint.
or it could be the painter ever put on a prime coat, if so scrape off any loose paint refill, prime and paint.
The Machines can mix to a different hue, from the same machine or shops. if you read the instructions it says mix two or more tins together so that all tins will be the same colour.
So yes there is a difference.
All tins from manufacturer will be the same.
what you've discribed is ok, but it depends on the pattern on the paper. you could put paper on floor and cover were you think the ends against the walls will be to judge.
knot, (bin can be used for this), spot prime (mdf), prime, (wood) stop holes, spot prime again, gaulk gaps, two coats of satinwood. all depends on your wallet
you could try cutting lumps off a thick pile roller, or tying cloth to roller. or thick paint then sponge in a plastic bag couple of coats till your happy with finish
Depends what can fit under treads, small palm sander might fit.. then i would use a couple of coats of osmo floor oil, very fine sanding paper between coats. it is called between coats.
https://www.diy.com/departments/plumbsure-chrome-effect-radiator-pipe-cover-accessory-pack-l-110mm-dia-15mm/1790075_BQ.prd
you could try something like this. also comes in white.