Now "lo sirji be abu bei by cu me lo sirji be abu bei cy" is fine, "the points aligned between point a and point b are among the points aligned between point a and point c". As you say, there is no individual "lo sirji" in this universe. "lo sirji" always refers to an infinite number of points.
The problem arises when you say that "lo sirji is a non-individual". It is not. lo sirji are just points, not "an" anything. You may call it a non-individual in some metalanguage, but in the language you can't, because there are nothing but points in the universe, nothing else. "lo pa sirji", in this universe of discourse, is nonsense, because the only things that can sirji cannot do it alone, they must always do it collectively in infinite numbers. There's only "lo ci'i sirji", "the infinite number of points aligned between two points". If you want to quantify over segments, for example if you want to say something about two segments, you are forced to move to a universe of discourse that has segments in it.