A DPS Simulator

[EasymodeX]

How autoattacks work, what their potency is, and if/how they are affected by skill speed

That valk website has a formula.

Essentially it's a "potency"-like coefficient based on the weapon damage, but the formula on the site scales it to a per-hit delay. A per-hit delay probably works better for your simulation though, so.

Edit: Don't believe they are affected by SS but not 100% sure.

If someone could test the different delays from when you start an action and when the buff/DoT is actually applied

That sounds super-obnoxious, time-consuming, and lag-sensitive . In the interim I'd suggest something random like 0.7s or 1.0s (or have the user input their assumption). Stop-watching some video recordings could work as well.