Installation is pretty straight forward. Grab the tint and a spray bottle with water and a few drops of liquid detergent. Spray glass and tint, apply, and use something flat to get the bubbles (and water) out. That's pretty much the 5 second explanation.
It will hold up just fine as you figured. I tinted the sliding door from my deck to my kitchen with reflective tint to cut back on the sun glare during the day, and it's held up really well.
I'm not sure how it'll work on already tinted glass. I'd worry that it may possibly cause some clarity issues. The darkness will of course change, but I'd be worried that it may make the picture a bit "mucky" with 2 different areas of tint. It may work perfect for all I know though, so I'd still give it a shot.