Have you ever tried to install the terrible interior doors Menards sells with the MDF casing? (yea we ordered 3 on sale not understanding the casing was made out of a thick cooked noodle - if the doors were a circle this would be a great idea)
I am asking because if you use the 1/4" drywall to go over a ceiling the MDF door is the only way you can easily up your installation frustration.
I have done two rooms in my life using 1/4" over existing drywall. One was after removal of popcorn ceiling the other was a bedroom that had severe water staining but existing drywall still in good condition. Will never do it again.
- 1/4" will sag even with screwing on 16" OC. You will have to use a normal screwing schedule but you will go through about a tube of adhesive per board. The glue will keep it from sagging given flat surface.
- As for screwing you have to get really good at evening up your screws to edge and then you go back in later to pop them all in slightly on low speed.
- If inspected some cities will not pass 1/4" over existing.
- It really is the equivalent of the MDF casing. You will be installing wet noodles on your ceiling. You MUST have a good helper to hold the other side of the noodle while popping the first few screws. I can put a 4x10 5/8" sheet myself easily (not fast but easy). I am pretty sure I would break a 1/4" sheet by myself.
- Due to the flexible properties of the drywall it may not flatten out when going from smooth to texture.
I think you understand the other parts - the longer screws, you aren't floating your edges here, and stuff like that.
My recommendation (unless you just want a way to spend this house time) would be to skim coat the whole ceiling. Scrape off as much texture as you can and just skim everything. It sure seems like a PITA but it is 1/4th the work you are looking at.
If you aren't good with skim coating a little secret is you just do the best you can - get it at 90%. Put a coat of primer on it. This will seal things up. Then you fill in the imperfections and it is really easy to fill/sand these because the primer provides a harder surface to lay against.