Booking two one-way int'l award tix then converting to a r/t (ideally with a stopover

Hi,

I am thinking of doing a cruise in Europe next year. Obviously, a cruise has fixed departure and return dates. I am looking to use miles for F or C class on UA or partners. As a result, if I see free one-way inventory available that arrives a day or two before the departure gate, I want to grab it as soon as possible (330/331 days out). I would then try to do the same thing with the return flight (grabbing it when it first comes available, if possible).

My questions (as a Gold/MM MP member):

1) Will I be able to combine the two flights into a single r/t ticket (whether they are open jaw or arrive at and depart from the same city? Will they charge me for this?

2) If I combine into a roundtrip, I know that I can do a stopover on a roundtrip award ticket. Would UA charge me for this:
- if my original booking have a stopover city and I just change dates to get a few days there?
- if I keep the same departure and destination cities but change the routing to get a stopover city? (Obviously the outbound or return dates would change to accommodate the stopover).

Thanks in advance.

John H.