re: I agree with almost everything you said
If you are right then the MARSIS data is maybe nearly meaningless as I've been told it penetrates to depths of ~3 miles (4.8 kilometers). And some say that is an over optimistic number. It won't find what it's looking for if it's sounds are to shallow.
Another problem is at 7 to 10 kilometers (4.35 to 6.21 miles) depths drilling methods we have used here on Earth probably won't work. How deep is the deepest oil well on land? Probably not more than 5 miles.
Still that just means more time before any worries over possible contamination.