...although there are seashells on mountain tops...
Yes. In fact, there are even “mountains” that are seashells. It’s called limestone formations. You cannot just find some few seashells on top of mountains, some mountains are seashells.
And yes, there are even megatons on top of megatons of seashells on the bottoms of oceans.
There is also very likely microscopic life residue, yet to be found, on the Moon which came from Earth’s rocks that have been ejected from the Earth to wind up on the Moon by meteors impacting to Earth.
The point I was making is that there’s not really any Scriptures that says these fossils were placed on mountains (or the Moon) via the flood or that mountains/valleys/cannons were formed by the flood either. But there is at least one Scripture that teaches us mountains formed via part of God’s creation events (not His destruction event, Noah’s flood).
Prov 8: (ESV)
25 Before the mountains had been shaped,
before the hills, I was brought forth,
26 before he had made the earth with its fields,
or the first of the dust of the world.
27 When he established the heavens, I was there;
when he drew a circle on the face of the deep,
28 when he made firm the skies above,
when he established the fountains of the deep,
29 when he assigned to the sea its limit,
so that the waters might not transgress his command,
when he marked out the foundations of the earth,
30 then I was beside him, like a master workman,
and I was daily his delight,
rejoicing before him always,
31 rejoicing in his inhabited world
and delighting in the children of man.
What a mighty God we serve, indeed. And to think, He cares about you and me, "delights" in us.
I certainly do believe that Noah and his family were the only humans to survive the flood just as the text says they were. And God causing all the animals that He commanded to enter the ark (which the text never says was a sample of two from each and every species on Earth) is no less of a miracle than the causing of the flood itself. Or for that matter, creating the Earth, Sky, Mountains, Oceans, Animals and finally his “delight”, Man.
I don’t think however that we, as Christian Bible defenders, are doing the text any favors or conviencing any non-Christians, by stating (or asserting or implying) that sea fossils were placed on top of the mountains via the flood. Frankly, I don't see God delighting in that type of assertion either. Though I'm sure He's quite patient and understanding when some do.
The much bigger miracle is the mountain itself even being there, and us gazing upon it, not a fossil or two or megatons of them being there.