So, if defender's unit get retreated/scattered and if it get rallied it will again fight as defender?
Yes. When a unit is in the "rallying" state during the turn, it will return to "stationary" state by the time combat is calculated, so if it belongs to the region owner, that will cause the region owner to be the defender.
1. Will battlefield be taken after single turn pass without battle?
I don't quite understand this question. What do you mean by "will battlefield be taken"?
2. Will the retreated/scattered unit fight as defender regardless of when it get rallied (even if it get rallied in day or two)?
As long as it belongs to the region owner and there's no takeover going on, yes. It's a fairly simple calculation, pretty much just using the situation at the time as its determining factors; it doesn't know how long any particular group has been there, so long as they're not freshly arrived, or how long it's been since the last battle.