I've looked at your images again and came to the same conclusion. Please take a look at your image of the bus. I have no doubt that the left hand image (labeled as a bare ZE 2/100 MP) is much sharper.
I do see some noise present on the LHS of your most recent comparison. (Look at the edge of the monitor bezel.) This looks very like the signature of a slightly over-sharpened image. I can't tell which has more detail.
That speckle will certainly be reduced when the extender is applied - but this does not automatically mean that there is more information present in the resulting image. For example - if an image was 2x larger but the spot size was 4x larger, it would appear far smoother but actually contain far less information.
I understand that the 2x would give you twice the effective distance at the same image scale. The problem with this is that image scale is largely irrelevant. Once again, the important thing is to maximise the amount of information present in the image after capturing and processing it to the same scale. Since the extender clearly blurs the image (quite a lot by my observation), it really can't be helping you.
(For what it's worth, do I own a ZE 2/100 MP and a 1.4 III tc. In the interests of science I'll do an experiment or two tomorrow. I'm not expecting magic though... some matches are really not made in heaven.)
Well I still don't see that, we'll need a third opinion
All images have exactly the same sharpening in lightroom (55, 0.8, 35, 5 masking). Zero noise reduction.
Something to keep in mind though: the bus image was taken from slightly different angle as I had to reframe to fit the 2x pic. This means the DoF area does not fall exactly in the same place. In that same bus pic, take a look at the bottom left corner, I see clearly that the in focus zone is sharper than the corresponding in focus zone of the bare Zeiss. Additionally I think the TC actually reduced the magenta cast of the bare Zeiss and the blacks are blacker. Slight, but for some reason clear to me.
The 2x certianly reduces contrast but compensates in resolution vs pulling the non TC pic to the same magnification. I still believe in overall the TC wins if you want to reach 1:1 magnification, that's all.
I have the 1.4x II as well but being only 40% more my feeling was the bare zeiss will be better when upsampling to the same level so went directly to the 2x IIIl.
I'll hope you can post your findings with the 1.4x, for the time being I think I need a few hours of sleep :=)