Hello,
1) The resistor specified is the internal impedance. The stability has been checked for those resistors.
2) Backup time T can be calculated as following : I backup (~9uA)=C * (Vbackup-VMBLO)/T
3)both caps can be charged. You need to make sure that the end of charge voltage, and the charge current are within the capacitor specification.
most of the supercap have a higher cap but a lower voltage. charge cap have an higher voltage but a lower cap.
If a long time constant is not necessary then cap can be selected because they are less expensive.
Best regards,
Flore