I hear an excellent penetrant is a 50-50 mix of acetone and ATF. Try that for a day or so.
Then try an electric heat gun on the dropout before applying torque and impact.
I like the idea of an impact driver on the DS with a Torx bit driven into the axle. I'd try an electric impact driver since I have one. I also have an old hammer impact driver and it's worked on larger stuff like car suspension parts.
Do you think the axle is seized on the NDS too? You might be able to break or check that by slightly spreading the dropouts, just half a millimeter.
Drilling the DS might not be too bad. You have a centered pilot hole and you should weaken the axle enough before you touch the threads to break the thing out of there, then you'll have access to file and pick out from both sides and run a tap through.