It also has a mains adapter. It is not necessary to operate it with batteries only.

No. The pairing of the units is fixed, no other units can be integrated.

A similar question has already been answered here. It depends strongly on how many walls, etc. there are up to the 7th floor. The manufacturer writes: "The reception range is up to 300 metres outdoors and up to 50 metres indoors. The reception range of the baby monitor changes depending on the environment and factors that cause signal attenuation. Wet and damp materials cause so much signal attenuation that Further down in the downloads you will find the "Manual". There, information on signal attenuation is listed in a table.

This should normally not be a problem. Please note that depending on the type of construction/wall thickness of a building, the reception can be weakened and can therefore vary individually from place to place. However, as long as the whole thing does not have to go through 10 walls to reach the receiver, I do not see any problem with these baby monitors with radio system.

No, inserted rechargeable batteries are not charged, but both rechargeable and non-rechargeable batteries can be used. We have had good experience with rechargeable, "always-ready" batteries, which do not lose their charge over time. Most of the time the unit is connected to the mains and in between it is operated by the rechargeable batteries. Recharging is thus rarely necessary.

Yes you can!

No, that's not possible!