It's kind of ironic that both in cosmology and biology we know an incredible amount, but yet we still don't how the universe or life on earth began. Sure, we have educated guesses, but until we can unify QM and GR or produce a cell from simple elements, they remain guesses.
There are numerous possibilities for how life began, and whatever process was involved probably took a minimum of millions of years to lead to something self-replicating and stable enough to start adapting. This makes abiogenesis theories likely impossible to directly test over human time scales.
Not that we shouldn't try... it's possible that certain conditions can give rise to self-replicating molecular systems that can adapt very quickly. This doesn't prove that these conditions were the exact ones responsible on primordial Earth, but it does prove that the phenomenon of abiogenesis is categorically possible under plausible early Earth conditions.
There are lots of other phenomena in nature that may be impossible to directly test. Biological evolution for instance is somewhat testable, but only on a small scale in both time and organismal complexity:
We can also run computer evolution experiments that validate some of the theoretical assumptions underlying biological evolution in a very abstract way, but these can't validate specific biological hypotheses about physical systems.
That's how lots of science works. Especially science involving the past, or science very far away.
Even if we do create a "cell" (whatever that means), it will only give us a possible explanation of how it happened on Earth. It will shrink the possible probabilities of all other methods, such as Aliens engineering us, but we cannot reduce all probabilities to zero... without a time machine.
Unfortunately we live in a world where even 100% definite in-your-face proof will not convince a frighteningly large %age of humanity.
IMHO claiming everything is "still a guess" shows a lack of understanding of science. I'm not judging, because I can't tell what you do and don't know from just three sentences. But it kinda sounds like it.
