Internally there is a lot of difference between the 1197c Si and 1198c Si units. Operationally there is not.

Yes, you can use an Ethernet cable between the 898c Si and 1197c Si units and share the GPS Receiver that is connected to the 898 unit. However, the 898 will have to be powered on for it to be able to share the GPS Receiver. An option to this would be to buy the AS-SysLink cable which allows two units to use a GPS Receiver without the other needing to be on. The SysLink cable would not allow them to share any data between the units like an Ethernet would but I thought that I would mention it as an option.

Yes, if you upgrade the software in the 1197c Si unit it will have the Di sonar capability just as an 1198c Si would.