ἢ γὰρ Κανδαύλεα ἀποκτείνας ἐμέ τε καὶ τὴν βασιληίην ἔχε τὴν Λυδῶν, ἢ αὐτόν σε αὐτίκα οὕτω ἀποθνήσκειν δεῖ, ὡς ἂν μὴ πάντα πειθόμενος Κανδαύλῃ τοῦ λοιποῦ ἴδῃς τὰ μὴ σε δεῖ. ’
Either take me and the sovereignty over the Lydian after you have killed Candaules, or you yourself must die straight away, so that…
Why is there an ἂν in there? What’s it doing?
Is the μὴ going with both the participial phrase (πάντα πειθόμενος Κανδαύλῃ) and the main verb (ἴδῃς), or only with the latter?
Hanging out with its friends. 
This is a regular feature of Herodotus’ style (and Homer’s), to use ἄν in purpose clauses with ὡς and ὅκως (= ὅπως). Found sometimes in Xenophon, but not otherwise in Attic prose, though of course Attic plays contain it as a poetic touch.
- Is the μὴ going with both the participial phrase (πάντα πειθόμενος Κανδαύλῃ) and the main verb (ἴδῃς), or only with the latter?
I believe it scopes over the entire clause (i.e., ἴδῃς). First, I have a hard time drawing good sense out of it that way. Second, the pattern ὡς ἂν μή seems quite likely to stick together. I can find fewer examples of ὡς ἄν… μή.
“So that you do not in the future see what you should not by obeying Candaules in everything.”