Is using a humidifier to make paint dry slower a good idea?
Altering the dry time (shorter or longer) is something I would generally not advise.  As the water evaporates the paint cures.  Altering the dry time interferes with this process.  This can lead to bubbling, cracking or even the tint separating from the base.  Now, are you going to run into problems for sure?  Probably not, and if it works for you by all means go for it.  I worked as a painter for years and there are two methods I have settled on that work for me.


This is probably the best for someone that is not overly adept at painting because it requires less speed.  Cut (do the edges with a brush) the entire room; ceilings, walls corners, around switches vents etc and let it dry completely.  Go back around the room with a roller making sure your last pass on a section of wall is a floor to ceiling un-interrupted pass (this helps eliminate roller texture being visible when it dries).  The last thing to mention here is roll as close to the edge as possible overlapping with your cut work otherwise the texture difference between the cut work and roll work will be glaringly obvious (particularly with light shining on it).  Edit: oh and if you are doing 2+ coats, cut multiple times first, then roll multiple times.  The more layers of roller texture over the cut work the better.
The second method involves either a second person (one cutting one rolling) or moving much more quickly.  This method works essentially the same except you move around the room in chunks and you want to roll over the cut work before it dries.  This is my preferred method, it looks nicer as the coat is more even (opinion).  Edit: and to be clear, ignore the edit above for this method...


Beyond that my standard painting disclaimers, use good paint, use good brushes and rollers.  Good luck.