Will UA sell SFO>FRA>Italy ticket with both LH & UA?

Hi all

Trying to book SFO>FRA>Italy. I can book SFO>FRA with LH on UA site, but for the return, it'll only allow FRA>SFO with LH and it won't show the option of UA. Unfortunately the FRA>SFO LH flight is at 9:55, and the Italy>FRA flights arrive in FRA in the afternoon (plus I wouldn't want to fly out of Italy at 6am to catch a 9:55 in Frankfurt).

How come it won't show the UA option for the return?

And if I do need to overnight in FRA, then would UA allow for SFO>FRA>Italy and then a layover in FRA for 2-3 nights at the return?

Thanks
Michael

ps - is LH 380 "C" angled seat very bad? I know it's not lie-flat like UA C. But how much worse is the angled seat compared to UA? I cannot find a current LH / UA C comparison thread.