1. Sure...you can apply it anywhere you want to - no-one here can stop you...Now whether it needs
it there or not is the question we can
answer and the answer is no you don't need a sealer on porcelain tile. On grout yes, absolutely! So apply it with a small brush on your hands and knees...and don't waste your money.
2. You'll probably get some on the tile. Just wipe that
3. Two coats, with a 24 hour lapse in between and you will have sealed your grout against staining from red wine and such, plus given you some protection against dirty wash water from being absorbed into the grout.
That's the best you can do. We do the same...