Sounds like you've made a decision not to cover any of the brick. Just to answer your question - the masonry adhesive is applied to the brick surface (not the Gyprock) in blobs approx 450mm apart. It sets pretty hard so to remove it at a later time you'd probably have to chisel it off. Any residue left in the surface of the brick could be touched up with brick coloured paint. Apparently you can also use cornice adhesive. Same way of removing & hydrochloric acid would remove any residue.

We recently updated an apartment to sell and got great feedback from the buyers on the paint - Dulux 1/4 Lexicon low sheen on walls and Vivid White high gloss on doors, picture rails and trims.

I agree with both your painter and your Google result. The issue is something that has soaked into the surface of the old wall, particularly as you had an odd result when cleaning. You can’t expect the painter to sand and paint as part of the original job but they may be able to offer advice and action if you’re prepared to pay them. If you want to DIY I’d be sanding the wall and trying some stain locker undercoat.

The best way to cover dark is to use a grey tinted primer first. Talk to a paint specialist. Dulux on line may help