Just few answers to (I hope) help you,this from experience with family properties.
1.Minimum 99 years.
2. As most Mortgage Lenders will not lend on properties under 99 years,then yes.
3.
I don't think that the lease extension would be any cheaper by doing it collectively,and of course you all have to agree to it,and to the amount for the extension.Personally I would do it on your own (if you can) then you won't get involved in other peoples leases.
This is only a personal view,not a legal one.