There's several Pentax FA 28-105's on eBay at the moment (as well as the current DFA version), but apparently not the PZ version which tend to be quite scarce. I think optically out of all the old film lenses around that focal length the FA 24-90 is about the best (unless of course you want the power zoom version to use that feature!) The 24-90 has the bonus of providing a useful increase in width whereas the loss between 90 and 105 is minimal.
Why not look out for a second hand DFA28-105? I doubt the difference in cost between a good PZ 28-105 and the DFA would be that great. (I've noted cheap PZ 28-105's but invariably they're for spares or repair.)

Should anyone be after a Pentax A* 1200mm there's one located in Redcar available on eBay.
link
Price not too bad at under £5K (by a penny.)
I'd love it but don't think i can justify it! Not sure what I'd use it for other than weight lifting practice (8.5 Kg.)

Seems a lot of fuss. Goodsync just does it!

It does, but the filter might not be exactly 10 stops so test it first to get the exact difference required.

Further to that, suggest taking a flat image such as a brick wall at various focal lengths and see what that produces.

Looking again at these #1 could be subject movement causing the blurred edge to the head (birds are like that!)
#2 looks OK at web size
#3 is presumably beyond the point of focus (if this is the whole frame and it should all be in focus you'd have a problem, but don't believe that to be the case here.)
#4 is partly OK, is it the whole image or a crop?

Have a look at GoodSync. I'm using an old, albeit paid for version which still does all I require. I chose that as nothing in the free category came close. It wasn't expensive and has proved very reliable.
If your needs are modest perhaps their free version might be sufficient.

Also, have you considered Windows Briefcase which may have come with your computer? https://en.wikipedia.org/wiki/Briefcase_(Microsoft_Windows)

What are you finding the free editions are missing?

Anything here:
http://www.amateurphotographer.co.uk/round-ups/accessory_round_ups/best-nd-grad-...
But just look at some of the prices. The Hoya circular... from around £210 !
Are you sure it isn't worth spending half an hour getting to grips with Elements? A no brainer in all honesty!
The only cheap option is to buy one of the Chinese ND Grad kits, but results can be mixed, though might be worth it to see if it's going to work for you before buying something more expensive.

If not an SR issue could it, perhaps have a loose or misaligned lens element in it? Given the problem has been narrowed down to the lens I'd ask Sigma the question. https://www.sigma-imaging-uk.com/support
A nuisance that happening. Suspect it might be emotional when you get the quote, but hope not!
I'd say give the lens a shake to see if it rattles, but given it has that optical SR that might be misleading.
Has the lens had the mount done? Could be an excuse to get that done and just mention the problem for them to check?? You never know, they may just sort it for you!

Just give it half an hour to have a play for starters. you may actually prefer it! With practice you might find you can process the image in moments. Just need to learn it!
If you go that route, take the images in raw as jpegs won't provide sufficient dynamic range. Pentax is one of the best for good dynamic range!
http://mattk.com/why-graduated-neutral-density-filters-are-dead-to-me/