DS doesn't seem to me to be any more guilty of building up a character one way only to find out that the reality is quite different. Such devices are used over and over again in writing, I mean, a classic example would be the wizard in
The Wizard of Oz. No one could say his reality matches the build-up. And, yes, one can say that even though he was ultimately very different, he served an intricate/essential part of the story by giving the scarecrow et al. what they needed, but the same is also true of Claude as [spoiler]he got Roxanne to speak, which destroyed Angelique,[/spoiler]which was most certainly an intricate/essential part of 1970PT. It just also happened that after he served his main purpose, he was killed off - but that's hardly surprising in the DS universe, no matter the time period or time band.
The actor playing Claude North struck me as being too young to be playing this timeless ancient being.
I don't believe the Claude we see in 1970PT was meant to be timeless and ancient. In Ep #1047 they did show the gravestone of a Claude North who died in 1866 - however, later on in Ep #1055 that man was explained as having been the present Claude's grandfather - and I don't think we have any reason to doubt that.