When you say "every living thing" I assume you probably mean every living thing on Earth.
Of course. I thought that was obvious. I am very confident that life has also emerged on multiple other planets but I find the idea of panspermia to be unconvincing.
is it really out of the question that, deep in the ocean, or in some other inhospitable habitat, life has arisen separately from the rest of life on Earth?
No it's not out the question. It would be a fascinating discovery. It doesn't change the simple fact that every living thing ever discovered from humans to bacteria to blades of grass all evolved from a common ancestor over millions of years. Not one single species or anatomical feature or biochemical pathway requires an intelligent design explanation.
In a sense even a creationist could agree with the statement, since they believe that God is the Father of all living creatures
Only if they wanted to indulge in equivocation. Most of them do that.
The you might want to qualify "ancestor" through reproduction of one finite living thing from another
No thanks. The meaning is obvious to everybody except a pedant or somebody who wants to misunderstand.
So Darwin himself wasn't necessarily dogmatic about this supposed central "fact" about evolution
Darwin left open the possibility that what we would now call LUCA was made by a creator although I think he changed this line in later editions of his book. He did not allow for divine interference in the process of evolution.
is it out of the question that new discoveries will undermine this conclusion and that in fact life on Earth had multiple starting points.
No it's not impossible that the very deepest roots of the tree of life have more than one starting point. If it is ever possible to show that to be true it won't change the fact of evolution.
Your final paragraph is woo woo.