Patching plaster, in my experience, is hard to work with because it sets up hard and fast. I have better luck using drywall compound to do the patching. I don’t know why you need to make the crack terribly wide; that will just make it harder to fill. Compound will adhere anyway. It might be best to dampen the cracks first, or just use the compound thinned a bit. Expect to go over the cracks a second time, but using the compound is fast.

I have never used “Goodbye Cracks”; might be a good idea. But just paint will cover small cracks. You will need to prime the patching with a latex primer like Zinsser’s Bulls Eye 1-2-3.