PT67.S2.Q20 - astronomer: earth was bombarded

Columbia Bound

https://7sage.com/lsat_explanations/lsat-67-section-2-question-20/
I am having a lot of trouble seeing how C and D are not saying the same exact thing. Here is how I broke down the structure of the argument.

Premise: Mars escaped severe bombardment by asteroids.
Major Premise/Minor Conclusion: There could have been microbial life on Mars prior to there being such life on Earth.

Premise: Many meteorites originating from Mars have landed on Earth.
Conclusion: Life on Earth may have started when a meteorite carrying living microbes were carried here from Mars.

Most explanations for this question say that D is wrong because it does not establish the truth of the main conclusion, saying that just because there was life on Mars does not guarantee that a meteorite carried life from Mars to Earth. But the thing I am having trouble understanding is that both the minor conclusion and major conclusion account for the possibility that they are not true by using could/may. So in order for the both conclusions to be true, all you would need to show is that it is possible.

After all, if the minor conclusion is true, if there is a possibility that there was microbial life on Mars, isn't it certainly true that this allows for the possibility for a meteor to carry such life to Earth (which is what the main conclusion is).

Columbia Bound

Also another thing I am confused about is how are C and D saying different things? C says some justification is provided while D just says justification is provided but doesn't specify how much. I take that to mean the same thing as some justification.

Also C says "establishes the conclusion" while D says "establishes the truth of the conclusion". Once again, I'm not sure what the difference is between the two. If you establish a conclusion doesn't that make it true?