You cannot combine separate bookings into one itinerary. If all the bookings are with UA and there is a misconnect on separate itineraries, they may take mercy, but they don't have any obligation to do so. This is somewhat risky, though. Personally, I have done things like this, but with more than two hours connection time. When IRROPS have occurred, I have been able to find alternate routings that allow the connection, and UA has made sure I got onto them. This was done with status.
Separately, if you wish to book different fare classes on outbound and inbound, assuming there are combineable fares filed, that is fine.