I am guessing you would be ok with the first scenario but not the 'SIN-SFO-SMF-SFO (stopover) SFO-DEN-DFW' one.

Just call and upgrade the 'DFW-DEN-SMF'. When you check in, tell the agent that you're using SWU and ask them to put a note in your SIN PNR (explaining the 24 hrs rule if they resist) so you won't be asked for it when you check in the next day. (Then get a printout of the PNR showing that, make a copy of your SWU with the serial number)