DoTs are on a Global 3s tick timer. As you said, you have a DoT with a timer of 15s, so you'd get 15/3= 5 DoT ticks. But, as it's on a global timer, your DoT ticks could be at 1,4,7,10,13s... 2,5,8,11,14s... 3,6,9,12,15s. Because of how this works, you have a chance of "clipping" a DoT tick each time you reapply it and your chance increases the closer you apply it to 3s, within that 0-3s window. Applying a DoT tick at 1.5s left means you have a 50% chance of missing out on additional potency from your last DoT application, which could of been spent on another ability. This only matters in the long run, and because of this, Skill/spellspeed is valued a lot less.

Buffs in this game are "snapshotted". The only buffs that apply are those which you currently had up when applying the DoT.

And Poison Potions are pretty cool. I personally don't use them, progression groups love them. Up to you if you want to use them.