You may find this question could be better answered by a specialist in Heideggerian philosophy at https://philosophy.stackexchange.com/, but from a purely linguistic point of view:
Neither "The modes of being in human is..." nor "The modes of being among human is..." read correctly (without the context of a philosophical text using the term 'human' differently to normal English usage). I see a few ways of correcting the grammar of the sentence:
Firstly, is is wrong; are agrees in number with modes.
Secondly, since in normal English usage human is a countable noun, it will need to be pluralised or have an article. Since it's being compared with a plural (non-conscious objects), we would choose a plural.
The modes of being in humans are fundamentally different from modes of being in non-conscious objects.
The same construction would apply with either preposition in or among.
The modes of being among humans are fundamentally different from modes of being in non-conscious objects.
As for whether there's a meaningful difference between in and among in this case, well, I really think that's a philosophical question, not a linguistic one. Depending on what flavour of philosopher they are, they might give you an answer framed in set theory, or group membership, or any number of other frames, but since your example sentence appears (to my non-philosopher's eye) to be Heideggerian, perhaps that's the place to start.