Is there any limit to the number to the number of developments that can be in any one area?

No, the only limits that are applied to developments are that without card effects (Such as 'City Gates' or 'Wake the Mountain') you can only play one a turn.

Q1: Can Developments ever be used once they are played face down (i.e. can they be flipped or taken back in the hand ever?)

Q2: Can Devs (or units) ever be moved from one zone to another?

Thnx!

Yes to both, due to card effects, see below for examples:

1) Orc race has a tactic card called: Rip Dere 'Eads Off!
(Card Effect) Action : Turn one target development faceup. If it is a unit, leave it in play and sacrifice it at the end of the turn. Otherwise, sacrifice it immediately.

2) Empire race has a tactic card called: Will of the Electors
(Card Effect) Action : Move up to two target developments from one zone to another controlled by the same player.

Other than card effects, there are no general game effects that can cause what you have asked.

So, Just to make clear... Once I placed down a card as development, I can't interact with it, only if stated by a card?

Because it was a bit confusing, if we may still use that card, say remove it as a development and play that card in the same zone...

As Ruvion said, you can just leave them in play as developements, unless a card effect allows you to do otherwise.

A card that becomes a development ceases to be anything else. You can no more just pick it up and put it in your hand to play it than I could do that with a card in my discard pile. Ditto for just deciding to pay its cost and flipping it over. Any of these things would need to be spelled out in the rules, and since they aren't, you may not. If there is no card effect outling an interaction the rules do not specify the only thing you can do is gaze fondly at your development and wish you hadn't played it as such.