Basically I am a PvE Demo Lock and simply can't decide which major glyphs will be best... Any suggestions?
What ones do you like?
Pick those. It is open to what you like. I tend to swap them out all the time.
Only possible required one is imp swarm for burst phase/ extra damage phases.
I recommend Imp Swarm, and perhaps Glyph of Demon Hunting should you want an 'oh shi-!' button if your tank dies, and to help solo some content. It's really just up to you. That's the great thing about the new glyph system.