I'm not an expert on alphabetization, but AFAIK kids actually have some trouble getting the letters "the right way round", I think especially if they start very early. They might be learning the abstract shape first and the learn to force it into the self-relative forms like always-left-to-right. Which kind of makes sense, since objects in nature are usually encountered from all sides, orientation is rarely valuable. So in that sense I'm not convinced it's really an expense to have to learn it. And anecdotally, my son when he was 3-4, when presented with a grid of items would tend to go over it row by row in alternative directions, like writing in boustrophedon. And as you say it would also make very long lines of text more readable - I think Greek inscriptions often have ludicrous line lengths, like several meters. So there it's not just your eyeballs that are jumping, you actually have to walk back to the start of the next line!
kids are excellent at picking information because their brains by default will attempt "wild" interpretations of the same information. Their brains are the perfect startup in a way, in that they fail quick, they fail often , and they learn immediately.
This is also why its hard to do magic tricks with young kids. They will tend to be looking at all the wrong spots. Their brain doesn't pick up on attempted misdirection, it just decide to make a new test happen from a different vantage point.
