Canon no longer stores the shutter count info in the image files. Instead, you have to download (and possibly purchase) software that connects to the camera via a USB cable to read the value from the camera directly. Where and how the data can be accessed changes with each generation of EOS DIGIC processor, so be sure you find software that explicitly lists support for your EOS T6i (a DIGIC 6 processor, camera EOS 750D).

If you have a MAC, you might consider this for-sale app that includes your camera type: https://apps.apple.com/us/app/shuttercount/id720123827?mt=12

This windows app is free, but I don't see your camera listed and the comments make it unclear if the EXE or the source code is being shared: https://sourceforge.net/projects/canon-eos-digital-info/reviews

Yes, a few thoughts:

1. Why not just manually focus the lens onto the subject as best you can? Watching a dot in the corner of the view finder can't be any more efficient than just focusing on the actual subject with your eyes via the view finder.

2. Why not use the back camera LCD in Live View and manually focus on the subject as best you can via the 10x zoom of the display?

3. Why not mount to a mirrorless camera and use the EVF in the 10x zoom and focus on the subject as best you can?

4. What aperture are you using, older lenses rarely look good wide open. What did your results look like at say f/8 and f/11?

5. Why are you wasting your time with manual focus lenses if you want autofocus confirmation?

Check the compatibility chart in pendennis post. One of the issues with "compatibility" is that many teleconverters "extend" into the back of the lens when connected together. Therefore, the back / rear of the main lens being "extended" needs space within the lens interior allow the teleconverter to be connected.

The chart from pendennis doesn't seen to address the newest autofocus AF-S Teleconverter TC-14E III, but if an older TC works for an F-mount manual focus lens, I don't see why a newer model wouldn't also, still being a manual focus combination. Maybe there's an issue with communicating the aperture control given the TC sits between the camera and lens? On a DSLR this may be problematic for focusing. On my mirrorless bodies with extenders and manual focus lenses, the digital body can meter based on the light hitting the sensor, even with the aperture closed down to the desired aperture. DSLRs tend to struggle with the aperture anywhere other than wide-open for focusing, as the DSLR doesn't dynamically 'brighten' the viewfinder to match the exposure of a stepped-down manual focus lens.