here below my answers per points:
1) In the HSNM Gateway configuration you have to add all the Mikrotik custom commands to configure your existing configuration. So when you import che .rsc file script will import your custom configuration too.
2) We never tested Mikrotik mAP but it should works fine. The only difference between other Mikrotik router is that this one has a flash memory so you have to copy the hotspot folder, downloaded with the .rsc file from HSNM, into the flash folder and not simply in the file list.
3) The choice of Mikrotik model depends on the number of users:
a) The RB9xx series is ideal for dozens of users;
b) The RB2xxx series is ideal for hundreds of users;
c) The CCR series is ideal for thousands of users.