Tub surrond caulking question
Number 1 

It could cause a problem, however it it unlikely. Use the shower a few times, and see what kind of condensation settles on the wall above the tiles. If you can see visible droplets of water beading on the wall, you will have a problem over time. As your house settles, and expands/contracts over time the grout will develop fine cracks. These droplets will over time go behind your tiles, and weaken the structure of your wall. If it was cement board it probably wont even matter, and we are talking decades before damage here, but why take the chance for a $5 tube of caulk? There are several brands of caulking meant for the bathroom that can be painted. 

Tip - when you apply caulk and run your finger over the caulk to get a smooth surface, make sure your finger is wet first. Makes a huge improvement to how the caulk spreads and looks. 

Number 2

The silicone will affect the drying of the grout, and you will probably find it takes well over a week to match the color of other grout. Depending on how good the seal is (i.e. is the grout exposed to air or a porous surface underneath), it could take a very long time. I wouldn't worry about it other than for cosmetic reasons.