The 'Lightroom history' of the edited file resumes (aka starts) when the edited file (PSD / TIFF / DNG) returns to LR. Not sure how / why it could be otherwise. If you retain the layers in the PSD, you have the layer-based history if you take the PSD back to PS. If the PS work was the 'finishing', you'll just have a catalog entry in LR for the new file and then any subsequent exports, etc.

The history doesn't 'span' new versions of the file. The history doesn't even come over to a Virtual Copy entirely within LR, just the current snapshot of the edit settings, starting a new history of the new file.

You seem to be going in multiple parallel directions with multiple incompatible full frame bodies. Do not modify the FD 135. Do not get an adapter for use of the FD lens onto a DSLR. Rather, look at scrapping all the unrelated DSLR equipment and consolidate onto the EOS R platform. Different adapters will be needed, but the FD lens can be used as-is on a mirrorless body as well as your existing EF / EF-S lenses. If might be a bit early today as Canon is about to rock the industry with a new EOS R mirrorless body, EOS R5 at 45MP. Three other full-frame mirrorless models are due to be announced too in 2020.

If you can't wait, then maybe another parallel track is needed. I have a Sony a7II with a Metabones adapter exclusively for my FD lenses to the Sony e-mount. The in-body stabilization and 10x focus peaking on a 24MP body make the FD lenses much more enjoyable than ever on film. Here's an example of the FD 135 f/2 on the Sony.

Blue Morpho Butterfly by Paul Sager, on Flickr
https://live.staticflickr.com/904/27712863998_e656e119b2_b.jpg

Consider whether they need 'high resolution'. For your intended print size, that would be files roughly 2400 x 3000 pixels for prints at 300 ppi (pixels per inch). Are you just dumping this issue onto the family members, or should you add value by sizing the pixel resolution to the intended use. This post provides multiple examples of the process, with screen prints, at the bottom of the post. Recommended resizing parameters for digital images

The best approach when attempting to learn LR is to go to the Help manual found online at Adobe and read anything on the Library Module. This will save you hours of problems and frustrations. Knowing how the program operates in this module is essential to your success in using it. After that you can rely on other types of help, videos, books, etc. I like the Martin Evening book on LR as he is very rational in his approach, a sort of A to B approach. You will love working within this program if you read the Help manual first!