1) Yes, you can pay part in points and part in cash/credit.
2) Sort of. You don't get the 20% off the entire ticket. You just pay 20% fewer points for whatever portion you pay in points.
Here's points only for a random search from DEN-ORD:
Here's cash and points:
On a points only redemption, I'm paying 80% of the cash value in points. On the points and cash redemption, I chose 15,000 points at random, but notice that the points are still 80% of the cash difference, $187.50.
Does that answer your question?
As an aside, it only costs 70,000 points to travel from IAD to NRT on United in economy class. If you get the CSP and a Freedom, you're already at 53.5k. CSP and a United Explorer gets you there with enough left over for a domestic ticket. Much better deal to redeem 70,000 points for the ticket than to redeem 40,000 points and pay another $1000.
[RIGHT][size=100]- Sapphire Preferred - Freedom - Ink - Platinum - Everyday Preferred -[/size]