Wainright in England thinks he found alien diatoms in the upper atmosphere. He published his results in a hack journal and I think he's getting beat up pretty badly.
Given: A rock containing actual extraterrestrial life falls from the sky and is discovered. Scientists find and isolate the life and correctly hypothesize the ET origin. How would they prove it to be true? All I can think of is if it had properties that could not arise on Earth, such as chemicals not found here or a structure that could only arise in a significantly different gravitational field.
Does your first paragraph lead to the second. i.e. Does the meteor that fell contain diatoms.
Diatoms evolved in Earth waters. If diatoms in the meteor were similar in chemistry and structure were similar to those in the rock then extra terrestrial origin could not be proven. There would have to be elements unknown as far as the periodic table is concerned and there are only 92 natural elements found in the universe and most ET material is Hydrogen. This actually means according to physical Law there are no other elements.
I do not know about Wainright but "thinking" he has found something does mean he found it. It is certainly possible that Earthbound algae could be carried aloft. Small spiders have been found in the upper atmosphere