Some people buy a keg of something they want to drink, then just don't take it back and eat the deposit... So, it depends on what the deposit is in your area. When I was buying kegged beer in Rochester NY about 5 years ago that was $55.
Agreed don't dump the whole thing...leave a gallon or even two if you're feeling real nervous, glass is pretty heavy compared to beer, keep your racking cane a few inches from the trub and enjoy your beer a few weeks later.
Well you need some trigonometry, which I'm bad at but if you put triangle helper into Google there are good "homework helper" sites you can just plug your numbers into...
My worry would be the 250deg heat limit depending on use, if it's for a mash tun or an E-kettle it should be fine, but if it's a direct fired kettle I'd be worried about the heat coming up the sides and melting it. I believe most use a borosilicate glass.
Rav