How acceptable is it to ask a professor a question such as "so you think it's justifiable to take points off my proof for this?"?
It's quite difficult to catch the tone of what you said from a written recollection, but:


  So you think it's justifiable to take points of my proof for this?


(emphasis how I imagine the professor perceived it) is indeed not very respectful. Next time, try something like this:


  I'm sorry, I understand that I should have given a more detailed reasoning about why [...] but it's just such a small and obvious part of the proof -- isn't it a bit harsh to take off Y points for this, what do you think?


Keep in mind that if you do this often you might still get a reputation as "that kid who tries to argue every tiny point even when they already have a pretty good grade" but if you are reasonable about getting told "no" it shouldn't be much of a problem. Still, you have to weigh your chances of getting an increased grade against the chances of being perceived as annoying or needy.

However, the professor's response in this case is completely unacceptable. Giving you full marks based not on your performance on the test but based on you getting out of their office is a gross violation of academic standards (even if they later rescinded this "offer"). Honestly, the whole thing that transpired here sounds like both sides messed up. It sounds like the professor (rightfully) immediately regretted "offering" you full marks.

So many people getting their grades increased sounds like the professor should invest a bit more time in designing a clear grading model and then consistently apply it. The way they currently have it set up pretty much guarantees having to argue every little point with every student. Even if what you said is inconsiderate and perhaps rude, they had themselves set up for it.