I do believe you can enter the level 20 heroic quests as either heroic or epic, can someone confirm, chacka??
That's partly right. The general rule is once you take Epic (level 20), all heroic quests are locked out (exception below).
In the previous seasons the lockout applied even to heroic quests that are level 19 on normal. So despite the fact that a quest registers as level 21 on elite, you could not do it when you're level 20.
There are three exceptions to the no-heroic quests rule: The Dreaming Dark, Tower of Despair, and the Lord of Blades raid, all of which are level 20 on normal, but also listed as heroic. So if you were to do all of the flagging quests for The Dreaming Dark and Tower of Despair, you should be able to do the final quest from the chain when you're levels 20-22. I never personally tested this, but I was assured by other players that they had done so.
There may be more peculiarities that I don't know about (this is DDO, after all), so there may be someone who has corrections they can offer.
Personally I hate the Epic lockout, which adds what feels like a weird, artificial barrier that destroys whatever level of suspension of disbelief this game allows when you reach that stage.