This is one sensitive case for sure. It's difficult not to agree that the "true" last film in the the series remains unfinished. But is the new version a sequel story-wise? Did they use the exact same screenplay for the new version? The determining factor is usually how it was released. Was the 1972 film promoted as the sixth film in the series?

Sorry to be so late in reply. There is only a limited amount of continuity after the first three films. Don Camillo and Peppone basically became stock characters in situations that reflected the times as the series progressed. The movies are all adapted from stories by Giovannino Guareschi, with the film in question adapted from his posthumously published and final Camillo novel. The thing is, the Fernandel 'Don Camillo e I giovani d'oggi' (1970) actually exists as a 35 minute short, albeit not listed here, and would much more correctly conclude the series (tt2296727.) I would categorize the 1972 version as a remake based on the same script as the incomplete film and not a replacement of that production. While clearly not a black and white issue, I think there is a compelling argument against its inclusion -- both by the moral weight of the talents' rejection and logically from the existence of the 35-minute short. It is the 1967 'Casino Royale' cash grab of the Don Camillo world, with a little grave trampling thrown into the mix.

Fernandel 'Don Camillo e I giovani d'oggi' (1970) actually exists as a 35 minute short

Where and when was it publicly released?

Well, clearly, it would not have been released theatrically at the time. As the IMDb page has the cover image for the Studio Canal release as a cover, I took it for granted it must exist as an extra on their collected series set (given the multiple ratings at odds with '72 film.) I'm afraid I neither own that set nor can I find a complete listing of the extras available on it. According to Italian sources, the 1970 filming lasted six weeks and was half-completed when Fernandel collapsed on-set while carrying an actress. When it later became clear that to him that he would possibly be unable to finish the film, he pre-recorded his remaining dialogue thinking it may be possible to use a double in the remaining scenes. I naturally presume there exists, or once existed, an assembly cut -- clearly there are production stills available -- but I cannot seem to directly confirm it has been publicly released at this date. Not sure where the IMDb sourced the 35-minute runtime from. Either its de facto existence, as a potentially lost film, would have to suffice or we would need to hope it makes an appearance at a later date. Nothing from the '70 filming was utilized in the remake as all the major players refused to participate; it is, at the very least, a completely separate film. Was hoping to provide you a better-grounded basis for making a determination, but I think you will simply need to use your best judgment here.

I'm afraid I neither own that set nor can I find a complete listing of the extras available on it.

I went down that road too, but it's not mentioned anywhere that I can find. It might only be a bad upload. The film can't be on TMDb without a release.