While my Revan always kills Bastila at the end of KOTOR, Shepard stayed true to Miranda. And that’s how you do it with Gibbed Save Editor for ME3:
The ME2 Bool for Miranda’s romance is 5209, it has to be set to TRUE. You also need to set the Init 266 to “5″. Do so before meeting Miranda the first time. If you already had done so in ME2, it will be properly recognized on character import and you can re-kindle the romance.
However, you still have to manually log in the romance for ME3, the Bool number is 21750: set it to TRUE. Do so preferably after meeting her the first time. I did so after the meeting in that apartment on the Citadel because I noticed that the romance variation of that scene only showed up when I replayed that scene as broshep.
If you use the Extended Galaxy Mod, you can check your cabin: as soon as the picture of Miranda is present the romance is properly logged.
This does not work for the casino scene: the option to seduce Miranda is replaced by “Girls’ Talk” regardless. Also, Shepard wakes up alone after the party and it is Joker who talks to her before returning to the Normandy (”a good ride”). Of course, you have a lot of unvoiced content and some occasions where femshep speaks with broshep’s voice. In case you usually play with subtitles off, you do not have to turn them on manually before meeting Miranda. ME3 shows the subtitles automatically if the audio is missing.