Fill Plug Tech: Cruel and Sadistic Method of Removal.
When I was 16, I couldn't get a socket type plug out of my VW transaxle. I too used the jack trick, but all I did was jack up the CAR! Had to be at least 1000 ft/lbs. 5 minutes later with a hammer and chisel, It was out.
I faced a similar challenge on a Volvo. screwed around with a fill plug for hours, then 5 minutes with a hammer and chisel.
As you noticed, the material is soft and brittle. With the chisel, it is OK if you pound off a piece or two of the plug. It is the FLANGE that is stuck, not the threads. By breaking off some of the problem, the impact vibrating things loose, and the rotational force you are applying, the thing will move a little, then 2 hits later will spin easily leaving you grinning all the way to the parts store for a new plug.
I too replaced all (I think) of my fill/drain plugs with allen head plugs.