I don't think the Warlock's Mystic Arcanum would count as "spell slots" for the purpose of these new blade pact invocations. However, a multiclass Warlock/Paladin could stack these invocations with Divine Smite, or a Warlock/Sorc could burn their higher level sorcerer spell slots. Either strikes...
On their own, I really like most of the new options. But some of the options opened up when multiclassing strike me as truly insane. As written, I see nothing in the rules to prevent a Hexblade/Paladin from stacking Curse Bringer with Divine Smite. And that goes for Fiend and Old One pact blades...