A342 Smell; touch; logical deduction?
A341 Any animal, including familiars (Warlock or otherwise) can only equip what items its form allows it to reasonably wear or wield. If it has hands, it can hold a weapon, though it wouldn't be proficient.
A339 I have heard that the grapple attempt still counts as an attack, even though you don't make an attack roll, and thus the grapple check is made at disadvantage if the target is invisible. But I don't have a source for that. And it may be RAI, but I can't see how it's RAW (as RAW, invisibility only imposes disadvantage on "attack rolls", which a Strength check is not).
Powers &8^]