But we am more likely to ship 100 humans + 1 million cells than to ship even 1000 humans to Mars.
My point is that 100 humans simply isn't enough, even with 1 million cells with them.
The iron womb simply isn't a good choice upon which to base assumptions - that development is bottle-necked by needing to be sure the kids it produces are okay. And for that you're stuck with the generation time of humans - you can't speed that up. Nine months to be born, and then at least decade to grow to the point you're sure enough the kid is normal to bet the species' existence upon the tech. By the time you know if the iron womb will work, it is too late to change the plan that requires it to be there. That's a problem.
implantation of fertilized eggs or iron wombs, same difference.
Go ask a pregnant woman if it's the same. That pregnant woman can't be risked on harder labors to build or maintain the colony in a harsh environment. Especially if you have at most 100 of them, and each one takes 9 months to produce a kid. That's a logistics issue. You need to have enough people so that with many pregnant, you still have enough to do work.
We ship fewer humans, but more cells, so we get our genetic diversity achieved better than just shipping humans.
There's no saying you can't send cells. Certainly, increasing diversity that way is a good idea. But while this is speculative, it isn't sci-fi - you're betting the continuation of the species on this. That means plans based on the most basic, reliable tech you have, not stuff you don't even know how to approach yet.
Send 10,000 people. If you plan for that, sending 1 million cells in addition will be child's play. And I assure you, the more actual people you send, the more cooperation you'll get from the folks left behind.