Gardz.

Here is a link that might be useful: Gardz

Agree with snooks. I would give GARDZ a try. Don't know if it would be beneficial to pole sand it b4 GARDZ or not. I'm sure someone else will pop in with suggestions.

This husband that worked for a painting co. Did he PRIME the walls first???

The walls should have been primed and sanded BEFORE the paint was applied. Gardz is a great product, but will do no good at this point. I would sand all smooth, clean, apply a pigmented primer ( Zinnser 123) and re paint.

A repaint can't be avoided, so decide if you have the energy before starting.

Prep

Walls need to be smooth and dust free

At this point, I would apply a quality primer with a quality roller. All things must start with quality in order to achieve a quality finish. Use a 1/2" nap roller cover. Apply the primer as new work, one gallon to 400 SF.

Let the primer dry for at least 4 hours. Back check your work. Any defects? Fix them now, sand and prime the fix.

Repaint using a quality acrylic enamel. Cover 400 sf per gallon and apply two coats. Don't guess at it. Measure 400 sf.
Use a quality roller cover, 1/2" nap.

Avoid painting in blocks, Lay the paint on top to bottom, and lay it off top to bottom. Keep that roller cover wet.

I don't understand what cheap drywall is. Isn't it all cheap? To me, this just looks like a bad paint job. Any repairs/patching you do will need multiple layers of paint because those areas are porous and will soak up the paint differently. This is the reason for sheen not being uniform. Also, there is a chance that you can apply paint with a short nap roller and not apply it properly or heavy enough so that you are essentally rolling the paint on with dead spots all over the place and this can create a very poor, uneven look, especially on high walls with a lot of natural light coming in from windows. Applying a full coat of primer will make the finish coats easier to apply evenly for sure.