Somewhere in the middle maybe? Does the slide exhibit that magenta cast?
However I feel this is more neutral (courtesy Photoshop Elements.)

I'm not saying if the adapter is any good or not, but the shake reduction still believes the focal length to be 300mm and act accordingly when it should be told it's 600mm.

The Pentax 1.4X TC is excellent and automatically adjusts the setting to allow for the change in focal length.

I take the shake reduction was turned off given the use of a tripod?

What happens if you try the lens again with the converter, which will have to be in manual mode anyhow?

Actually I think the problem might be with the electrical circuit where the lens has disconnected itself at the point. the contacts from the lens mount meet the lens 'proper'. A fairly common fault with some Sigma lenses where the mount contacts are only pressed against the lens contacts. Usually it fails just showing it can't read the lens aperture, but quite why its not showing anything in your lens I've no idea.

It might be worth trying to wiggle the lens mounted on the body to see it it shows an intermittent fault.
Try that in normal SLR mode rather than live view.

The only reason the lens cold have shorted out if there was power in the lens when it failed. The teleconverter without contacts in between would mean there was no power going into the lens from the body to cause damage. . Whether a charged capacitor inside the lens might do that I've no idea, but haven't heard of that happening before.

Does the teleconverter still work with other lenses and is it one with all the electronic contacts on it?
What happens if you try the lens with the camera in manual mode?

Not a 645d then?

Apparently sine early GR111's had a minor defect with in the 4-way controller., See:
https://www.pentaxforums.com/articles/product-updates/ricoh-gr-iii-service-notic...
If it's that follow it up as it sounds an inherent fault you shouldn't have to pay to get it fixed.

I suppose if no one buys them they'll all get dumped at the Council tip in a couple of days (ref ITV new this evening.)

If you look at the advert posters on the London Underground out of a carriage window, the 'pixels' are truly enormous, Of course they're not meant to be viewed that close. So long as the image looks fine at a normal viewing distance it should be fine for the purpose.

Surety the printing company will sort out any resizing issue? That's unless you're printing the posters yourself.

Suggest you Google Viewing Distance.