I'm afraid it may not be your last piece. Basically, you are trying to get the emulator to switch from an interlaced (frontend) to a progressive mode and seems you can't attending to your posts (never tried Demul myself since I believe it's still far from emulating properly anything, but the issue is quite generalized as you point out). You can try to solve it by letting Demul's devs know about 15-kHz usage so that they code it similarly to GM at some point or you can do it yourself right now at hardware level, by forcing the progressive scan.
Now it's your third post and you're still not mentioning which games you want at 240 lines under Demul, but, whereas CV-1000 games or DC's Black Matrix will work with the suggested solution since the hardware natively runs them at 320 x 240 (so you can run them at 640 x 480 and setting off one field, it's an easy and well-known concept these days, never fear), SF3 W Impact _may_ not work properly if Demul's authors aren't still aware that it uses a special mode which only Capcom used in vey few DC games (or they just don't give a ---steaming pile of meadow muffin---). That is to say, it may be an unemulated feature, hence my suggestion to check it beforehand, in case those are the games you're after -- it's uncharted territory, I guess, but you won't find many titles natively for 15 kHz under this emulator, so whatever.