The intention is that every reseller will get a separate basic IP (default) and optional additional IPs that he can use for special purposes.

All of these IPs shall be different from the server IP.

As a result: can we expect that the SSH access by IP is limited to the given reseller package so that he cannot see other reseller packages that exist in parallel and he never should see things you normally see if you are Admin of the total server.

Example:
server has IP 86.86.86.86, hostname xcs2.example.com, root access only for us as a provider. We have ALL rights.
Reseller reseller1 has IP 86.86.86.56 as a default, the hostname alias is reseller1.example.com, dns of domains with this reseller are on 86.86.86.56 as default and the primary nameserver is reseller1.example.com, the secondary nameserver is one we give like ns2.example.com. The reseller "reseller1" has root access but can only see something that looks like a server but only with his domains. No other things. (can he reboot the whole server?)
Reseller reseller2 has IP 86.86.86.57 as a default, hostname alias is i-net-hosting.example.com, DNS of domains with this reseller are on 86.86.86.57 as default and primary nameserver is i-net-hosting.example.com, the secondary nameserver is one we give like ns2.example.com. The reseller "i-net-example" has root access but can only see something that looks like a server but only with his domains. No other things. (can he reboot the whole server?)

Bottom line: the business cases you are looking for are
migration of two (or more) servers to one new server without changing all DNS and doing changes with the registries.
Structured level of reseller configuration on a server with multiple resellers. I.e., dedicated IP pool and hostname per reseller.