Um, I think your CBTar damage calcs seem a little weird. I keep coming up with 25% - 31% on MetalKid's calculator. I have heard MetalKid's calculator has some slight errors, but even still yours seems too low.
And regarding Sandslash, it seems like really the only benefit he has over Donphan is Swords Dance, and I guess X-Scissor for Celebi, but in exchange he actually does take hits alot worse than Donphan does to the extent that many moves that would 3HKO Donphan end up 2HKOing Sandslash, for example +Atk CBInfernape Close Combat/Flare Drive is doing 46% - 54% to Donphan, which is like a 3HKO 99% of the time, whereas that same Infernape ends up doing 53% - 62% to Sandslash, a guaranteed 2HKO. Another more worrying example is that CBMetagross Meteor Mash has like a 1% chance of 2HKOing Donphan, and a 100% chance of 2HKOing Sandslash. Now I'm not saying either of these two matchups are what make Sandslash worse than Donphan, they just show that the stat drop from Donphan to Sandslash actually does cause some tangible differences in hit-taking ability, for what seems to me like minimal gain.
Actually, now that I think about it, I think Sandslash's biggest advantage over Donphan is takinga 60 bp Grass Knots, which is actually really awesome, but yeah I still like Donphan better, because of his better hit-taking ability and Ice Shard. Like Obi already pointed out, Sandslash having Nightslash is pretty much cancelled out by Donphan being able to 2HKO Gengar with Stone Edge/Assurance.
edit: but I do think your analysis was very well presented :)