Freeing stiff nipples? I've got a nice used rear wheel that's slightly out of true.  Simple fix is true it up.
But the nipples are stuck to the spokes and won't turn freely.  I managed to get one spoke to do two full twists and still it wouldn't release.
The spokes are round profile, so there's no good way to hold them firmly while turning.
Without replacing all the spokes, what are my options to release the nipples?
I agree with Daniel’s comment, I’ve had success with using vice grips to hold spikes myself. Be careful not to use excessive force to prevent crushing the spoke. It may help to add a small “flag” made of tape to each spoke to help visualize spoke vs nipple rotation.
Matt’s suggestion of using penetrating oil and heat is a good idea, provided you removed the tube/tire and rim strip first like Daniel suggested. Brass does in fact expand more than steel when heated.
Of course, be sure to use a high quality spoke wrench to prevent rounding the nipples.