Advice on settings for cleaning up highly compressed video

[Selur]

1) Do you have any tips on when it could be recommended/not recommended to use BasicVSR++?

Using BasicVSR++ under 'Others' (sometimes masked, just for the chroma or just on some scenes) is mainly meant for restoration, the lower the source resolution the stronger the smoothing&co effects are (so sometimes it makes sense to upscale before applying BasicVSR++).
For cartoons with low details, I usually at least try the effects of BasicVSR++. It does strong cleaning and line enhancement there.
I would not use BasicVSR++ on stuff that is already high quality or on content where I want to preserve small details (at least not unmasked).

Is it possible to upscale in Hybrid using BasicVSR++ or is exclusively used for restoration?

BasicVSR++ under 'Frame->Resizers' is meant for upscaling, here unlike with the restoration a 'high' quality source should be used. BasicVSR++ here isn't meant to clean the source.

Cu Selur