I have some thoughts on this based on conversations with the designer TJ over the months. Keep in mind I don't have my game copy yet with the final rulebook or anything so I can't really back up what I'm saying right now, but here's my unofficial take-
I noticed the final campaign scenario rewards you with the Ooze Wand when you complete it. But what is the point of this 'reward' since there is no mission after that?
I think we can hopefully expect there to be additional 1 player campaigns in the future. Aside from that, I believe the real idea here is that now the Ooze Wand can now be slotted into any deck for 2 player play. Of course no one is stopping anyone from just using it without even bothering with the 1 player, but I think it's more fun to actually earn it.
Furthermore, is there supposed to be a list of cards you are not allowed to initially put into a 1-player deck, because you might just be rewarding yourself with one or more cards you already have in your deck.
Not that I'm aware of. You can tune your deck with factionless add-ins the same way you would for any 2 player game. The exception of course being you can't use cards from the "Mercenary" deck because you're playing against it.
Also, are heroes meant to also be used in the 2-player game? If we do so, are we supposed to just level them up to max before playing?
Yes, the Heroes are intended to add variety and flavor to the 2-player game, but strictly optional. As in you and your opponent agree beforehand- are we playing the standard game or spicing it up with Heroes? And they do not need to be leveled up.
Does that help clear up any of the confusion?