I'm conceding defeat on the sump plug.
There is not enough room to get the pipe wrench on and my twist sockets don't seem to be doing anything. My pipe wrench is 10-inch - maybe with a smaller 6-inch one you could get a hold but it still looks doubtful.
I did try a bit more aggression today and hammered on the 16mm twist socket (a size below spec). I got it most of the way on but although it was tight to begin with, it still ended up slipping off without moving the plug. I'd driven nine miles and waited about a quarter of an hour before starting and everything was still very warm to the touch.
Unfortunately, as much as I would like to have an operable plug, looking at it rationally it didn't make sense to proceed (and once again I was out of immediate options). I don't trust myself to try chiselling it and I had the extractor sitting there anyway.
So out the dipstick the oil came. It worked alright, not brilliantly, but then mine's quite a low-end one. It lost pressure after about 3.5 litres and I had to keep pumping to get the rest moving.
In total I still only got about 4.5 litres out though, including what dropped down with the filter. I'm sure the extractor's tube was at the bottom of the sump and the dipstick was coming out completely clean when I gave up but I thought there should be a bit more. I'm not sure this is encouraging with regards the prospect of long-term extractor use.
Due to slightly over pouring into the funnel I still ended up adding the book figure of 5.5 litres of new oil. After running the engine for a few minutes and letting it sit for 15 minutes, the dipstick reading now comes up to marginally above the top of the hatched area.
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Once I switched to "Plan B" with the extractor the day went smoothly really. Thankfully the filter came off as easily as it should - I was wondering if that would have been welded on too! - and no leaks after starting or sitting, so my first oil change is done at last. Now we can go into cold start season on nice new golden oil.
I also replaced the rubber seal under the oil filler cap having noticed a little oil behind the filler neck which I'm fairly certain I haven't spilt there. The new rubber (genuine) is much softer and seems to create a tighter seal.
Long term, I'm not sure what I'm going to do with the sump plug. Probably leave it for now as it seems to be for the best. I won't need to be doing this again until April or May anyway so I'll put it out of mind for the time being. I don't believe the extractor did capture absolutely everything today though so this saga may not be over yet.
Sump plug still in situ - maybe forever?