Keisei Skyliner & Metro Pass

I would like to buy One way Keisei Skyliner + 1 day Metro Pass (2600yen). But i have a few questions below :

1. where can i buy this discount pass? only in Narita?

2. Can i use the 1-way Keisei Skyliner pass on Ueno (or Tokyo area) to Narita instead of Narita-Ueno?

3. Must i disembark/embark from Ueno? Can i embark at Nippori instead of Ueno?

1. where can i buy this discount pass? only in Narita?

You can also buy them at dedicated travel agencies in Korea, Taiwan and Hong Kong and on board of Peach flights bound for Narita Airport. Otherwise, they are only available at the airport.

Can i use the 1-way Keisei Skyliner pass on Ueno (or Tokyo area) to Narita instead of Narita-Ueno?

No, I don't think so.

3. Must i disembark/embark from Ueno? Can i embark at Nippori instead of Ueno?

I think you cannot use the 1-way ticket in direction of the airport. When coming from the airport, you may disembark at Nippori Station.

I want to buy two ways Keisei Skyliner + 1 Metro pass (4500 yen) on 30/4/2014 at 16:00. Where can I buy it? Can I mark the seat in airport counter in first? Thank you for your helpful!

If you plan to exit at Nippori, tell the person who sells you the ticket. Otherwise the faregate might reject your exit. Of course, an attendant will override the rejection, but it's better not to have that problem in the first place. Getting off at an intermediate station on any ticket is not guaranteed to be trouble-free.