Is it true you can get 400 beads per T1 NM kill?
Don't know who told you that, T1's don't give close to that.
I heard you can get beads by popping and killing T1 over and over. How does this work? How much do you get per?
T1's give you 50 beads each per kill, then add on your luck vorseals. If you had 10/10 luck and killed one T1 you would get 60 beads. Now Blessing of Loyalty doubles your beads but only the base amount not the bonus. So with Loyalty up you would get 110 beads per T1 killed.
Farming T1's usually works best if you have a group of people that can quickly kill several of the T1's. I do it with my friend and some mules, usually me (BLU or WAR) + WHM mule + GEO / COR / BRD (friend and their mules), might add a "tank" if some LS member needs beads. I've done it fast with three and fast with six, doesn't matter as long as you can kill the NM's in a minute or less. Pop and kill in rapid succession, then switch leads and have them pop theirs, then switch again, and so on until you need to get another batch of pop items. Can farm a few thousand bead in the time it takes the dragon to cycle once, also have bonus stones and gear drops.
If you don't have any group resources then it's probably faster to just kill the dragon, assuming your server is doing dragons.