I don’t have the answer to your first question, hopefully someone else will reply to that one shortly. However, look at section 20.1 on this > Guide < do you have user groups configured, with bandwidth control in place?
Answer to Q2 :- When you upgrade the firmware it takes your current configuration and applies this for the new firmware. However, once the Firmware has been upgraded, when you reboot the device you do have the option to reboot to your older firmware version. Please note, changes made to your configuration in 7.1.0 will not be applied to the older firmware - as the “partition” used for the firmware retains the configuration settings that were applied to that Firmware version (if that makes sense). The screen shot below shows the option to reboot using 7.0.2, but if you go from 6.3.2 to 7.1.0 then you will have the option to reboot to 6.3.2.
If you do make a number of changes whilst using Firmware 7.1.0 and then decide to reboot to the older version, you can download a config file from 7.1.0 and then upload this configuration to the older version.
I assume you tested the remote user speeds when no other device was using the B20? There are no speed limitations of PPTP that I am aware of outside of bandwidth availability on the Balance itself.
However if your remote users are really remote (ie lots of hops between them and the balance WAN) the increase of latency will have a negative impact on bandwidth availability for those sessions.