Hi AIRchrist, and welcome back. I'm no expert with Burn, but I've been playing the deck off and on for some time now. I do however have some ideas regarding your questions:

Quote Originally Posted by AIRchrist View Post
- Since I've read numerous articles about fetchlands not being all that helpful in mono-colored builds, would it behoove me to run straight mountains(and maybe a Ring or two) to avoid life sacrifice for a negligible benefit? Mind you, I'm not in love with GLM, so I don't really care about filling my graveyard.
IMO it is not necessary to play Fetch, so going for all mountains is a valid option.

Quote Originally Posted by AIRchrist View Post
- SDT is a card I'm interested in running as a 1 or 2-of, and I would think it would be rad with Thunderous Wrath as well. Is SDT considered an added benefit to Burn(outside of the obvious pros), or should I focus more on spells that actually play towards our objective?
SDT helps you set up your draws. If you are planning to play SDT, however, Fetch comes in handy to give you a fresh three cards to look at.

Quote Originally Posted by AIRchrist View Post
- I noticed that our old staple of Flame Rift has been cut. While I know it doesn't feel great using it with Eidolon, wouldn't it be a good addition to Burn, even as a 2-of?
I have cut it entirely from my deck. I think it is a matter of preference.

Quote Originally Posted by AIRchrist View Post
- Do you feel that I need to even keep GLM at all?
Again, preference. I change my main from time to time. Sometimes there are two, and sometimes I split them between main and SB. I like how it can be the last bit of damage, takes down little critters, and cleans up your GY.

Quote Originally Posted by AIRchrist View Post
- I'd like to run 1x Volcanic Fallout, but don't see many sweepers in SB's across the internet. Is there an argument against boarding a sweeper?
Big creatures are everywhere, and swarm decks are not always present. This is also meta-dependant: if you have lot of Grixis or Elves, well, a Volcanic Fallout is an option.

PS: Thunderous Wrath is bad indeed, and 'good in Modern' does not mean it is a valid card for Legacy perse I would trade it for a 4th Fireblast.