Can I keep unused paint in the roller tray?
As long as you can get it airtight, it will work just fine.  I use the disposable tray liners and try to get as much of the paint off the liner as possible, pouring the remaining paint back into the can.



I leave the liner on the roller tray, cover it with plastic wrap and then put the whole thing in a tie-off garbage bag, trying to get as much of the air out as possible.  Keeping it in a cool place helps to cut down evaporation too.  

Another tip: 1 gallon Ziploc freezer bags are the perfect size to hold a 9" roller cover between uses:



I squeeze the roller cover out with a painter's multi-tool, drop it in the bag, and seal it up.  It won't keep forever, but it will certainly last a week.