I have a special Voodoo hammer here, it might help, but it might make the lens less able to focus on its own too.
What is the chart like WITHOUT the Tc attached, just for comparison. Don't move the camera, just stick the chart up, shoot WITH the Tc, then take the Tc off and shoot again without touching anything, other than focus. See what the results are. If they remain soft at the bottom I would think there is a plane shift in the bayonets and the workshop is your only recourse, with BOTH sent in for optica matching.
D810 + 600mm f4 + 1,7TC ........ same lens and TC as you
Image straight out of camera - FULL IMAGE - NO CROPPING - NO PROCESSING - Fine Tune set to ZERO - all in camera defaults on standard and all at 0 ................... just converted to 250kb jpegs in LR and reduced in size to 950 longest side ........(originals are 44MB RAWs at 7360 x 4912)
Cloudy, no sun this morning - flat light - dull but OK - could have been just a little better
single cell focusing and spot exposure - focus on eye
Cirl Bunting taken using LiveView
these are what I would expect and can be improved with just slight processing
F8 - 1/1000th sec - 0EV, (I would usually use +0.3 in shots like this)
Good tripod, gimbal, - if I shot at speeds below 1/1000th sec, at my ability level, I would not expect to get a sharpe image with the D810 .. and I am surprised I did so well at 1/1000th sec
obviously the set up works, but as I said only effectively at f7.1 or f8 ......... all the better if the light is good
no idea why you are having problems if when manually focussing you do not get a sharpe image - tests against targets are are well and good ...... but actual shots show what the set up can do (and I believe that my lens/TC need fine tuning)
As this was my TC i feel i need to comment. I bought the TC from Dale Photographic as a used item and only ever intended to use it on my 70-200mm F2.8 VR. It worked a treat with minimal affect on the AF (was used for motorsport). I can supply pics to prove this if needed.
Unfortunately i don't own any of the more exotic lenses like some peeps do on here so i would've had no idea that there would be any issues with Daniels lens.
I sold the TC honestly and accurately to my knowledge and would like to offer my sincere apologies for the issues Daniel has experienced.