Pro Milone 47

primum certe liberatur Milo non eo consilio profectus esse ut insidiaretur in via Clodio: quippe, si ille obvius ei futurus omnino non erat…

I understand the sense of it, but how to explain the first non?

My take is that it modifies eo consilio.

First surely Milo is acquitted that he left not with that plan of &c.

Now that makes perfect sense. Maximas, amice, tibi gratias.

Libentissime!

The non eo consilio…ut construction is used elsewhere.
From Cicero de Oratore 2.138 (or Liber Secundus XXXII)
Haec ego non eo consilio disputo, ut homines eruditos redarguam;
I treat of these things not with this intention - that I might refute learned men;
Or, I treat of these things with this intention in mind - not to refute learned men;

From Cicero pro Milo 47, our topic sentence
_primum certe liberatur Milo non eo consilio profectus esse ut insidiaretur in via Clodio:
quippe, si ille obvius ei futurus omnino non erat…_Firstly and resolvedly Milo is acquited of the charge of having set out not due to the intention
to waylay Clodius on the road, for of course it is obvious, in as much as that one was so not
on a path where he would end up meeting him.
Milo is not innocent because he does not have a plan (who can prove intent?).
The reason Milo is absolutely innocent is not because it can be established that he did not set
out with a nasty plan in mind, but because the implication that he did so is absurd from the start
(primum certe) - the two characters could not possibly have met on the road. It is quippe that rhetorically
begins the negation of the accusation.