5, 1, 17

[Constantinus Philo]

θάρρει, ἔφη, ὦ Κῦρε: οὐδ᾽ ἐὰν μηδέποτε παύσωμαι θεώμενος, οὐ μὴ κρατηθῶ ὥστε ποιεῖν τι ὧν μὴ χρὴ ποιεῖν.
This oude is used in the meaning of kai, so why not just using kai?

[phalakros]

οὐδ’/μηδ’ ἐάν is the negative version of καὶ ἐάν/ἐὰν καί.

[Constantinus Philo]

yes but here the preceding clause is not negative, so why oude? it is probably adverbial meaning 'even'. if so, then ou me is redundant. οὐδ᾽ ἐὰν μηδέποτε παύσωμαι θεώμενος, κρατηθῶ ὥστε ποιεῖν τι ὧν μὴ χρὴ ποιεῖν. is good enough

[phalakros]

The preceding clause (θάρρει) has nothing to do with it.

Yes, it means “even” and introduces a concessive clause. Positive concessive clauses can be introduced by εἰ καί or καὶ εἰ. Eg. εἰ καὶ χαλεπόν ἐστιν, δεῖ ποιεῖν ἃ χρή (“even if it’s difficult…”). It’s “positive” because the main clause (δεῖ ποιεῖν) is not negated.

Negative concessive clauses are usually with οὐδ’ εἰ or μηδ’ εἰ. Eg οὐδ’ εἰ πρόχειρον εἴη, οὐκ ἄν εἴη ἐφ’ ἡμῖν τοὺς νόμους παραβαίνειν (“even if it should be easy, it would not be in our power…”). It’s negative because the main clause (οὐκ ἄν εἴη) is negative. In Greek it’s idiomatic for the negatives to repeat (οὐδ’…οὐκ). Similarly in the sentence from Xenophon. In a language like standard English, the repeated negative would either be superfluous or the two negatives would cancel each other (since standard English does not have negative concord). But it’s proper idiom in Greek. Your rewriting, removing οὐ μή, is neither idiomatic nor grammatically possible (οὐδέ + subjunctive in an independent clause like this is not grammatical).

Does that make sense?

[Constantinus Philo]

now everything is clear thank you

[Hylander]

For οὐ μή + aor. subj. see Smyth 1804; Goodwin GMT 294 ff., Appendix II -- here, emphatic denial.