-Not recommended for a number of reasons. Never mind the issue of burnt fingers handling the hot glass
-Keep them in a dry location so the pins don't oxidize. i.e. avoid cold damp garages and basements.
-1000s of hours, and in some cased tens of thousands. Their emissions drops off with use, but i've never managed to wear one out. Its really not an issue worth worrying about.
-Same as an old light bulb.
-Don't be afraid to use it, but i wouldn't leave it running unnecessarily as you are wearing the tubes out over time.
-Yes, as long as each tubes is the correct part number you can use any brand you like.
-The smaller tubes does the amplification, i.e. makes the music signal louder, and the larger tube provides the power to drive the headphones.
Enjoy!