Hi Sandra,
1) Hana is a must for XS installation as it is a part of HANA and not something that can stand apart. It is within HANA. Best explanation is this pic
2) HANA XS is a small foot print sever, as per my experience it can replace other servers.
3) Only thing I can find is that it is a part of HANA and cannot be used out of it as an independent one. The whole purpose of it's existence was to serve for SAP based Application.
4) There are road maps available but just need to google for that.
Hope these answer your queries to a certain extent.
Regards,
Mahaveer Jain