Certainly artificial light flickers 100 times a second in the UK, especially fluorescent light (incandescent lamps even out the flickering much better) - that fact can used to calibrate the speed of turntables, so any shutter speed faster than that will be affected to some extent. Whether colour temperature also flickers I do not know, but your pictures would suggest that is the case. Try using 1/100 s (I know it won't freeze movement as well, but it will at least identify the cause of the problem)

Jacobs are a pretty reputable firm, and they have a chain of real shops in the UK as well as an online store.

From the penultimate picture your sensor needs cleaning!

Even using the distortion correction in the Pentax Photo Lab?

Has anybody actually tried this with success? Is the distortion at the corners (e.g. off shaped faces) when you do this any diffferent from that produced by ultrawide rectilinear lenses? I also am contemplating an ultrawide and I quite enjoy the fisheye effect (I have the K17 which I use with film) but it is not always desirable. If a reasonable result can be obtained by de-fisheying I might go that route.

Park Cameras? They are usually very quick to get new Pentax stock.

For reasonable money you could have the Pentax 50-200 or the soon to be released Pentax 55-300, or the Sigma DG Apo Macro 70-300 or the Tamron Di II 70-300. All are a more or less direct replacement for the 70-210. I can vouch for the Sigma 70-300 and the Pentax 50-200.

The 18-250 is a very useful lens and a pretty decent performer given the inevitable compromises - I've had no problem with enlargements to A4 from it.

On some early third party lens the flange by the aperture control lever is of a diferent size or in a slightly diferent place which makes it impossible to mount it on some more modern Pentax bodies - comparing this image with a pentax K mount manual lens (specifically my newly acquired SMC A 135 2.8 ) seems to suggest this is probably your problem.

I want one! it would replace my 16-45. OK I lose 1mm at the wide end but I gain SDM, 50% more reach and it zooms the 'right' way so won't cast a shadow with the built in flash at wide angle!

In my defence I did leave that complicating detail off my compatibility table! (I just used the word 'probably' in reference to vignetting)