I think it'd be fine to limit the additional rules to:
1 Attack cards you play do not affect your partner
2 You add up VP at the end.
So IGG, Raid & Noble Brigand are a tad worse, but whatever. Saves all that rules issues.
I admit the ability to have your partner gain your cards is fun, though. Maybe you can still have that fun by creating specific cards? I mean, it won't pay to have them actually printed, but it'd make a nice thought experiment. Here we go:
Collaboratory, 5$, Action
Your partner draws a card.
Coworkshop, 3$, Action
Your partner gains a card costing up to 4$. (Yes, I'm aware this is basically the OP rules implemented on per-card basis)
Strategist, 5$, Action-Duration
Discard your hand. If you discard any cards this way, at the start of your partner's turn, they get +5 Cards, +1 Action, +1 Buy
Baroness, 4$, Action
Your partner gains a Duchy.
Penduler, 5$, Action
Put this in your partner's discard pile.