Video artifact removal and restoration - How can it be done? (Really?)
If I understand you correctly, then most of the restoration goals will be impossible to be reached. While my knowledge of existing videos tools is not extensive, I am still pretty sure such a software does not exist. As a software developer I have a good grasp of what is impossible and what is possible and what effort you need to do that.
When details are lost, they are lost. How should an AI restore these? Well, any AI has to be trained first and this needs to be done with pairs of a master video in highest quality and its heavily-compressed export in the target codec. The problem is that the export video has less information than the master and there is no way to define a sensible bidirectionaly mapping. Imagine the master contains a scene with a machine with little screws. Another scene shows a machine with rivets. But after a heavily compression you cannot see a difference between screws and rivets in the export video. Due to loss of detail, both screws and rivets now look like grey smudges. How should you decide what is correct if you only have the compressed video? You can only make a guess. And that's what the AI would do. This means you get a restored video that often shows quite different details than the original video, however now and then the AI will make correct guesses too. This is of course theoretical, I highly doubt there is an AI based software for this job as it would be extremely expensive in development and with only a few potentional customers the price tag would be enormous.
Discolouration is also hard to solve because often there's no easy way to differentiate between intended de-saturated areas and compression artifacts.
Macro-blocking can be fixed in theory by applying a blur effect. But you have to sacrifice sharpness. Fine artifical noise can be added to gain a bit of sharpness again. Anyway, this can be done and such a function is probably available in some tools.
Colour bending can be solved in theory without too much effort but I don't know if anybody does offer such a thing.
Update: As has been pointed out in the comments, DaVinci Resolve can do de-bending. Both Premiere and Resolve use some AI based tools to restore a video to fix dust, scratches, de-interlace. But if I infer correctly this does not help much with your problem of restoring a heavily compressed video.
All in all, you cannot undo the artifacts of heavy compression.