So, a more regular Attic-prose way of expressing the meaning rendered by πιθοῦ would have been πεῖσαι?
Yes, I think so, but a very quick and dirty check didn’t yield any examples (aor. act. inf. πεῖσαι crops up frequently). Perhaps the register of the aorist middle imperative of this verb is poetic/dramatic.
This seems remarkable! (I take it that you meant middle, not passive).
I mean: would not this (if proved true after double-checking) be a decisive argument in favor of πιθου?
I take it that you meant middle, not passive).
Yes, middle. Fixed it.
would not this (if proved true after double-checking) be a decisive argument in favor of πιθου?
No, I now think πείθου is probably right, and there’s no need to amend it to a form that is generally not found in prose, and that seems inconsistent with Crito’s respectful attitude towards Socrates.
You can see the “correction” in the S ms. referred to in the OCT note on 44B 7 “fort. corr. nesc. in S” here:
http://gallica.bnf.fr/ark:/12148/btv1b11000123f/f22.item.zoom
It’s in the 6th line from the top (right-hand page), which begins -τες ετι και νυν εμοι π?θου
Use the + button in the upper left-hand corner to zoom in on the spot.
The preface to the new OCT tells us that S is loaded with errors (most of which aren’t reported in the critical notes) and was written by a rather ignorant scribe, but the δ “family” doesn’t “speak with one voice” and goes back to a manuscript that evidently included variant readings; and therefore S can’t be neglected, since it could possibly contain good readings.
As mwh wrote, the obscure “correction” in S at 44B is of no value for the text – it’s probably something not even worth mentioning in the apparatus – but I thought it would be interesting to take a look at it to get an idea of what’s involved in collating a Byzantine minuscule manuscript and editing a text. In all likelihood, however, if you were to examine the actual ms. (rather than a digitized image) it might have been marginally clearer.
It does look like either someone began writing πε but corrected it to πιθου, or that someone wrote an ε over πιθου. It’s accented as a barytone, but the accents look lighter than the rest of the text in general.
It could simply be an example of itacism. Noting that you are using internal criteria for your conclusion… 
It could simply be an example of itacism.
πιθου is a 19th century conjecture. Itacism usually goes the other way (ει>ι).
Noting that you are using internal criteria for your conclusion…
Well, so what?
But this is too insignificant an issue to waste as much time as we have wasted on it.
Whether it is significant or not depends on what one aims to learn. For me, this whole discussion turned out to be exactly the learning experience I needed, providing a window into the mind of the editors that I could not not find in textbooks.