OK, let me try to clear some of this up, largely but not wholly ignoring the last 20 or so rat-a-tat posts. (The number seems to be increasing as I write.)
- On the αν, a couple of points. First, as I think you realize, Tugodum, αν has no semantic force of its own, but affects the semantics of the verb. The meaning resides in the verb+αν combo. Second, “its semantic force in this particular sentence”: since λεγειν is infinitive, the Greek itself doesn’t reveal exactly what form it would take if it were finite. λεγ- tells us it has to be either present or imperfect, and αν tells us it has to be indicative or optative. The syntax (δοκῶ with bare infin.) tells us that it should be understood as 1st person singular. All that, combined with the context, indicates that either ελεγον or λεγοιμι would be appropriate, i.e. the meaning could be (roughly) “I think I’d be saying …” (ελεγον αν) or “I think I’d say …” (λέγοιμ’ αν).
I don’t address the following λέγων, on which I already commented.
Here’s an earlier post of mine on conditionals, intended to dispel the fog that tends to surround them:
http://discourse.textkit.com/t/conditionals-a-guide/14120/10
You or others may possibly find it useful.
- On επιεικη λεγειν. You can call επιεικη adverbial in that larger sense (as qualifying a verb) if you want, but it does not function as an adverb, and it doesn’t mean quite what επιεικως λεγειν would (“to speak in a reasonable manner”). επιεικη, just like αληθη in αληθη λεγειν, is an inner aka internal accusative (and likewise neut.pl.). I wouldn’t call it a cognate accusative (unlike e.g. λογον λεγειν) but it has the same construction.
What Smyth 1573 says may confuse (a lot of what Smyth says may confuse!), but cutting out his comparison of μεγαλ’ αμαρτανειν with μεγιστα αμαρτηματα αμαρτανουσι only makes confusion worse. His saying that the adjective in e.g. μεγαλ’ αμαρτανειν is “treated as” a substantive doesn’t mean that it is one, any more than his translating the phrase as “to commit grave errors” does. As to his saying it’s “ordinarily plural” in prose, well, it is: witness αληθη λεγειν, επιεικη λεγειν, μεγαλ’αμαρτανειν, etc. etc., while plain αληθες or επιεικες λεγειν or μεγ’αμαρτανειν would be out of the ordinary. Of course an internal accusative can be singular: λεγειν τι “to say something (something substantial),” αληθες τι λεγειν “to utter a truth,” επιεικες τι λεγειν “to say something reasonable,” αμαρτανειν τι “to commit an error,” ἓν αμαρτανειν “to commit a single error,” while “to commit just one grave error” would likely be μεγαλων αμαρτηματων (μονον) ἓν ἁμαρτανειν.
Tugodum, you say you were taught to translate a construction like μεγαλ’ αμαρτανειν as “to err greatly.” Leaving aside that “err” is not in current use (except in translations from Greek or Latin!), translation can conceal as much as it reveals. In this and comparable cases, it conceals the fact that μεγαλα is not an adverb (as you are well aware) but an inner accusative (as perhaps you are also aware). Smyth’s “to commit grave errors” is better, or “to make serious mistakes." The construction is the same as e.g. πολλὰ αμαρτανειν, to make lots of mistakes, as distinct from πολλαχως αμαρτανειν, to be wrong in all sorts of ways.
Sometimes we have to move beyond what we were taught.
Joel, you say you post here so that you can be corrected. That’s an excellent strategy for your own learning if you are then corrected, but otherwise it risks merely spreading misinformation, something I think we should try hard to avoid. Often I do correct you, as I did in this case, but I can’t correct you all the time. 
Anyway, I hope what I’ve written above will help you understand. You don’t need to look for a “set phrase,” merely look at the construction (bare adj. w/ verb). In the extract you quoted, ..τάχα δόξει λόγον τινὰ ἐπιεικῆ λέγειν· ἐπιεικῆ δὲ εἶπον,…, the first επιεικη of course agrees with λογον, a cognate accusative, while επιεικη λεγειν without λογον τινα would be neut.pl. (Cf. Smyth 1573 part-quoted by Tugodum above.) The follow-up επιεικη I presume simply puts the first in quotes, as it were: “I said ‘reasonable’ …”. But I don’t know the context.
If anyone has any serious questions about any of this, feel free to ask, and I may reply tomorrow or later. But I’d hope that this is adequate elucidation, and might be useful for more than just the two of you.