temperature of paint

Let it get to temp. When I used some cold gloss I ended up with curious "sag" patterns caused when it warmed up :-)

Should be fine.

If you are using paint pads you may want to experiment with diluting the emulsion with water at about 1:10 ratio. I've not tried it on ceilings, but on walls it goes on much quicker and (maybe surprisingly) gives a much more even coverage, with less need for a second coat.

I'm not sure how the regulars here view this, it certainly works for us.

A quick emphasis that I'm only talking about diluting the emulsion here.

For Gloss paint I've been recommended to sit it in a bucket of warm water for 30 minutes before use, stirring well to get it nice and smooth. Be careful when opening the lid etc.

Thick paint doesn't go on smoothly, so its worth making the paint slightly thinner before applying it.

Make sure that you have enough to complete the job. Eg just do the ceiling. Then do a wall then another. Wait long enough for each job to dry so you can see if it is in need of more coats.

You should be OK but there will be an obvious difficulty in getting the same shade on a new tin. If they are in steel cans they may need filtering through a cooing utensil r maybe a nylon stocking.

It wouldn't be worth using it if it has been frozen. I don't know for sure if it would be unusable I just wouldn't bother taking the risk at the price of that stuff.

A lot of pro painter mix their trade stuff with a little of the good stuff to get better coverage, so there is no problem in mixing even slightly different formulas.