You should find all you need to know here with regard to using FTM with Windows. The publishers are not flagging up anything that affects initial installation.

Yes FTM 2019 can be installed on both PC & Mac however you would need to use perhaps a cloud back up system to share your database between the two machines.

I would look seriously at Family Historian. Most programs allow you to start any number of files with a different tree in each. However in Family Historian it is also possible to keep track of different unrelated family groups using named lists but in the same file. For instance family A from area x could be identified separately from family B living in area Z. Having everyone in the same file avoids lots of duplication of work with other aspects of the program.

You may download a demo version of Family Historian at:
https://www.my-history.co.uk/acatalog/Family_Historian.html#DEMV3

No. However, two people who are working on the same family will tend to work on separate sections of tree and will swap and merge data on an occasional basis.

Yes. You may transfer data backwards and forwards using the GEDCOM file transfer protocol between PCs and MACs