Extending the Musjid

Q: Insha Allah, our masjid will be doing some extensions. We will be extending by putting an upper floor. I wanted to find out, if the ground floor has a masjid and a sehen, is it permissible for us to make the entire upper floor a masjid without having a sehen?

For example, if the bottom floor is 500 square meters (300 sqm is the masjid and 200 sqm sehen). Can we make the entire upper floor (500 sqm) masjid?

Q: What is the Shar'ee ruling regarding the building a multi purpose hall on the ground floor and a masjid directly above it?

Q: Our Musjid, has gone through an extension on the left side, However there is a separating wall (not completely separated few rows the an entrance, and few rows again) in between the Original Musjid and the extension. The extension is used for itikaf and is part of the Musjid. Usually on Jumma or when the musjid gets full the extension is used. The set up is similar to Musjidun Nabawai alhis salam. where outside the the front row continues, separated by walls:

1. Is the reward the same when standing in the front row,between the separating walls?
2. Should the original Musjid be filled 1st, then the extension or vice versa?
3. Can I stand alone in the extension to get the reward of filling the front rows?