Redo - This sounds too strong. Without the on-trash effect I would probably price it at $3 or $4; the versatility can be very nice. But with the on-trash effect, a chain of them is only once card worse than the equivalent chain of labs. That's clearly way too much for a $2 card, since it's very easy to spam a $2. I might price it at $4 or $5. I really like the idea though; it's like Procession in that it makes you want to gain things that you might not really want to gain.
Wagon Train - Seems fine balance-wise. I'm not sure how exciting it would be.
Scientist - I really like this, but I'm not sure it's okay at $2. In the long-run, it benefits both players equally, but delays can make a big difference in Dominion. Bandit Camp's delayed Spoils keep it from being +$3, +2 actions, which would almost certainly be too strong for $5; Caravan's missing the re-shuffle bump it down from $5 to $4. I think it should cost at least $3 so it's a little harder to spam, maybe even $4.
Historian - Interesting. It's a way to deal with green cards, and it's a green card, so it's like Island in that regard. It also helps block incoming junk, so that's another similarity to Island. It's cheaper than Island and it's not terminal, and it can block multiple junk cards over the course of the game, at the price of only being able to do it while you're gaining them. In terms of function it's probably better than Island, but it may be harder to get them to score. Another Golden Deck (I think a little better than the one you mentioned) is 4 Historians+any number of Market Squares (or Worker's Villages/Markets). You draw your deck every turn, then buy as many Coppers as you can, setting them aside on your Historian mat. If you have 10 Market Squares, that increases your score by 11*4/3=14 points per turn (more precisely, 15 points every two turns and 14 points every third turn). So actually that's pretty crazy, and could lead to some very long games, trying to drain the Copper pile. Maybe not though since the Copper pile is getting drained so quickly. Something to think about.
Inventor - Very simple, should be fine balance-wise. Very similar to Caravan, maybe a little better.
Reinvent - Not really sure I understand what this card is trying to do. The two things that it does seem unrelated. Now that I re-read Invention, I think I see it: you're supposed to reinvent invention to put it back into your hand, get another invention, and any action card that's not Peddler. Okay, now that I get it I think I like it a lot. It might compare favorably to some of the other non-drawing terminal one-card trashers, but most non-drawing terminal one-card trashers (is there a better term for this?) are weak, and this is distinct enough from them that it's probably okay.
Overall I really like these. I'm excited to see more.