I've had a similar senario where, a nipple head sheered off and left a spoke waggling about.
As a precaution after i had replaced said mangled spoke and nipple, i inspected them all to realise the spokes twisted as i attempted to undo them, basically all seized, like yours.
You see, i was in a position to just cut the spokes as i had ideas for the rim and hub elsewhere.
If plusgas hasn't helped, i expect freezing them may break the seal as it is steel vs alloy, should work, freezing does for seized alloy posts in steel frames. If you don't have access to a chest freezer, to leave the wheel in for a couple of days, I would suggest some freezing aerosol like this.....http://www.ebay.co.uk/itm/PH-Crack-It-S ... 48429368c7
This may prove labourious esp X 32/36 but just might work, maybe do 3 spokes at a time. You blast it on til a frost appears then, leave 2 mins (or whatever the instructions say).
p.s swap nipples and/or lightly lube each one at the same time as you reinstate them.
Ride yorkshire on an old bike.