The consumable slots (Alt 1 - Alt 3) do not accept powers at all, only Potions, Bandages, Food, Campfires. (Our slots are not combo slots that accept both powers/consumables, you get one or the other!)
Good question, the situation you provide isn't an apples to apples comparison though.
In order for the Assassin to be in a state of "DoT ticking + still in stealth";
a) they would first have to break stealth to apply the DoT and then wait the combat timer out (at which point the DoT may have already worn off) ,
b) or use a Vanish to get back into stealth while they have DoT