If it’s solid you have one side of breast warm other side cold causes condensation! If you know anyone cheap digital camera lower it down and look inside once air temperatures drop in evening
It’s a damp problem by either ventilation issue or could water be entering from external side maybe! Is the building solid or not etc! Maybe you have heating on and causing condensation
Looks like damp stains but obviously any entrapped water causes new plaster to dry differently which causes colour differences really if the marks were there before and it’s been reskimmed they probably just used pva instead of grit
Oh artex can’t be blown badly as your adding weight just not much really but if you pay for a plasterer they won’t guarantee work over artex I don’t think as I wouldn’t
If pattern not too deep skim over the top any little bumps take off with scraper but use ppe and seal entrances but if low count don’t worry too much but once dust settles hoover to remove traces etc
Cheap is fine mate just get the right type of metal depending on situation as what plaster your using but yes more expensive trowels are better but we use daily so worth the cost if your not experienced in plastering maybe a cheap trowel might be best and if you want to break in use a old...
Your best option as need mesh joins is run straight edges and cut slowly in layers with sharp blade then carefully remove old skim then pva the lot and skim but on the brown patches let pva set then skim or skim tacky
I’m a plasterer so depends on your experience really but mix yourself instead of ready mix and deep areas you want some sharp sand in as if applied to thick with soft sand may crack and could blow or build up in coats! Like previous said run rails if needed and maybe let them set and once the...