Also in regards to Ebolt, unless a new bug was introduced since Mangler/selos (which is entirely possible, but I tested it on Phinny, again on Agnarr, and again on Mangler in classic and it remained the same mechanic) it does not count as full dmg credit.
The way it works is that DoT's tick based on the server tick. So if your DoT was to land at say 1 second in to the server tick for example, you would not receive a dmg message until the server tick. What it does to compensate this, is that you get ONE free DoT tick upon the DoT landing, that counts towards your dmg credit without effecting the health bar of the mob. So for example with Ebolt, it's possible to do 1094 dmg in 6 seconds if it lands at the sweet spot of the server tick. The free tick you get counts as over kill dmg as well. So for example if a mob has 2k hp, and a necro lands an ebolt in that sweet spot server tick, even if you nuked it for 1257 dmg, doing 62% of the mobs health, the necro will get the kill.
This mechanic was part of the reason why I ran 5 necro's at efreeti on agnarr and was able to not lose efreeti's even to guilds that would sit raids on me. I was securing 51% of the Efreeti's HP in 6 seconds. Also part of the reason why a new meta in classic started to happen where necro's were just spam casting Ebolts in dps races, because they are getting that free 393 dmg every cast without speeding up the kill of the mob.
Now I'm unsure if a NEW bug has happened since, but this was how it has worked for years. It's just more known now because of DoT revamps making DoT ticks much higher, making it more noticeable. This has always been a mechanic present on the TLP's and even something I personally took advantage of early on Phinny.
Edit: Quick edit, even though this is the first TLP I have not tested it on since I'm not playing a DoT class this go, if it were true that some new bug happened recently that made 1 Ebolt count as full duration upon cast, it would be impossible to KS a necro who only casts 1 ebolt on any mob with 5K hp or lower, and there is not a lot of mobs in classic with more than 5k hp. I know I have certainly KS'd ebolt'd mobs this TLP that have less than 5k hp.