I completed statue of Dahomroc in varrock museum for the first time and my question is am I correct that i have to complete it again to get more replica statue pieces?

Second question is about construction. If I remove stairs in skill hall, do room upstairs still remain or they will be removed too and i have to build them again?

No you don't lose the room above. When you build a staircase upwards you are given an option to build a room above it.

 

The statue replica pieces can be collected as soon as you finish the statue and collect a replica piece. What happens is you'll be able to collect all the pieces again, but wont be able to use the last pair on the plinth until its been a full week since your last completion.

but how do you earn statue replica pieces, by skilling or by compliting statue in museum to earn replica pieces from archeologists in museum?

but how do you earn statue replica pieces, by skilling or by compliting statue in museum to earn replica pieces from archeologists in museum?

By completing the statue you get one replica piece.

It takes a minimum of 30 weeks to complete the statue in your home.