It's always going to be difficult to get an acceptable finish when you are trying to cover different types of paints on a wall surface. It isn't impossible but does take a lot more work than many people imagine because it involves applying multiple coats.
Emulsion paint is deceptively thick when it is wet but actually dries as a thin film which, as you have found, doesn't do much to hide previous brush marks or shiny surfaces. Multiple coats will do this or, in extreme circumstances, using lining paper to get a uniform finish can sometimes be easier.
You shouldn't be using a shellac-based sealer over a large area because of the high VOC content. In such cases, (for covering water stains) an oil-based primer is a better alternative.
Again though you are going to face a similar problem when it comes to coverage and it will take several coats of emulsion to match the existing finish. Thinning with white-spirit isn't advisable (to any great degree) because you'll just need extra coats to get the same stain-blocking effect.
If the affected areas are minor then an aerosol-based sealer will be easier to cover.
The main tip i can offer is to use a mini-roller to apply the primer and, when it is dry, to paint over these areas only with your choice of finish, again with a roller, until you get a result you are happy with. Then, and only then, repaint the entire walls to a final finish. Ideally, 2 full coats.