Do Eldritch Invocations stack per turn?

Can a warlock use more than one Eldritch Invocation per turn? IE Repelling with agonizing with spear?

Depends on the invocation.

In general, passive abilities -- those that you can use without taking an action, bonus action, or reaction -- can be used freely, and are usually always in effect.

So, your particular question about all the cantrip modifiers is a yes. Whenever you cast the relevant cantrip, you can use the invocations that modify it. You can also do this while using Devil's Sight to see the target in darkness.

But you couldn't also use Ascendant Step to levitate in that turn, because casting Levitate requires your Action, and so does every attack cantrip I can think of.