...Chekov's father to meet Chekov's mother several years earlier, meaning that he fathered Chekov years earlier, meaning an entirely different sperm met an entirely different egg, resulting in a different apperance...
Every egg produced by a woman has the same genetic material, and every sperm produced by a man has the same genetic material, except for the deciding gender chromosome.
By your theory, the only difference might have been Chekov being a girl.
There is, however, the matter of HOW the mother's and father's genetic material combine. This is why all the boys of a family don't look exactly like one another, and all the girls don't.
Each child is a random combination of their parents' genetic material.
This Pavel Chekov could be more or less a BROTHER of the one we know, except that the one we know is never conceived...
...or is born when he's supposed to be, but ends up with the Russian form of the name "Peter".
Another possible explanation for Chekov's hair is that after the Kelvin incident, the Federation outlawed any human wearing a vulcanoid sugar bowl haircut, which as we all know Chekov preferred.
In place of this, he had his hair permed and with 23rd century techniques, his perm is permanent.
Honestly, I think most of them were laughing at how bad and over-the-top Yelchin's accent sounded most of the time.
