(As a Rangers fan, I'm going to try and be as objective and fact-based as possible)
I certainly do not disagree with you. I think the main thing that will work against Kreider is the fact that he's older, and despite still being considered a rookie, had played in 49 NHL games prior to this season (including playoff games, which do not count in Calder consideration).
Not to mention he's quite a bit older than MacKinnon.
So as it stands now, in 5 less games, Kreider has 4 less points. Kreider also has the +/- stat by 5. This of course comes from missing a few games early in the season where the Rangers were absolutely dominated (Oh, Marty

).
Then there is the advanced stats, but I don't know how meaningful they are in regards to this selection.
Kreider has 37% O zone starts, and a positive relative corsi of 3.7% (5 on 5, not close or tied). MacKinnon Has 34% O zone starts, and a negative relative corsi of -1.5% (5 on 5, not close or tied). So you can take this to mean that when Kreider's line is on the ice, they are generating more chances relative to the rest of the New York Rangers than MacKinnon's line. Is this all Kreider or MacKinnon? Of course not, but the statistic is there.
In all situations 5 on 5, Kreider's relative corsi is +10.6%, second on the Rangers to Brad Richards. While MacKinnon's is +6.8%, near the middle of the pact when it comes to Avs players.
This coming on the basis that the Rangers are a HORRIBLE team 5 on 5 this season. Kreider's line is always making noise, and it comes with the success of CK.
So, I think if it came down to it, all said and done, CK and MacK having the same or similar point totals, the award is going to go (rightfully so) to MacKinnon. And while they are both tearing it up at the moment, I think MacKinnon's play has a higher chance of tailing off than Kredier's, who I believe will say more consistent, because Kreider does have that extra experience in the NHL.
Anyway, I'm biased. I'd love nothing more than to see a Rangers player take home the Calder. But I think I made a decent enough argument here.