A way to give to your dwarves nice rooms without pissing off nobles is to resize their room to 1x1, containing only the bed, ideally placed above a common stone floor. This way, the 'real' room can be as big and decorated as you want, they'll get the 'slept in a good bedroom' thought, and the nobles won't get jealous, since officially the room consists only of a bed.
If it's a matter of principle and you want your deserving dwarves to have royal rooms, nobles-be-damned, then yes, nominate as baron some dwarf you think you won't get attached to.
I know how one can get attached to the founders. For your duchess, as a last resort you could try in your older save to hastily make a mist generator (even if operated manually, you'll need it only for one instant) or expose her to the mist of a natural waterfall or something, while you reset the rooms and reassign them. Remember to check the 'R' menu, to see the quality of the room you currently have, if you haven't already.
In general, about barons: i never nominate any of the founders. I choose someone with a liking for minimum 3 craftable kind of objects that i'm not planning to export (because of the chance they're gonna prohibit exports instead of order production when they can order 3 mandates).
If they're asking for some metal i discover i don't have on the map, i try to scrounge at least 1 bar, so when they ask for production i can forge and re-melt a statue of that metal, given the 31.xx bug
(metal items requiring only one bar to be forged, and statues melted give back one bar).