Thanks! The clarification and extra information are appreciated.

The only point that you might get any argument on is what constitutes "landing enough probes to check for as many possibilities as we can before sample returns". We can in fact land robotic probes on every square mile (or even less) of Mars and take hundreds of years before doing MSR so you are rather lacking on detail there. What constitutes a reasonable amount of prework is open to debate.

Given how life works, I suspect that if (big if) there is native life on Mars somewhere, there will be native life on Mars everywhere. If some places are more hospitable than others, you'll just find more of it. While on Earth you have to look at a very specific place to find penguins, it's pretty hard to find someplace that doesn't have bacteria or other microorganisms, even in very hostile environments.

If a few probes designed to find biological activity land in widely separated areas and find nothing at all in those surface rocks and buried layers that would be subjects of an MSR, then odds of another probe, like a MSR, landing somewhere else and then finding something go down drastically. It wouldn't take very many probes to establish this.

Again, I figure in a thousand years the place will be crawling with Earth bugs anyway.