First you need to differentiate if you want to use the adapter for simple reading/loading (The Smart Data Access use case) or for realtime. If it is the first part only, then the location of the adapter does not match much as all it does is opening a connection to Oracle using the Oracle JDBC driver and translating SQL from the Hana syntax into Oracle syntax.
For realtime it is a bit more work as you need to make sure the adapter has access to the transaction log information in the Oracle dictionary.
Yes, you can install the Adapter on a different server, as long as it is of the same kind, Unix versus Windows.
Explained in the PAM and in help.sap.com