1: Yes the 999 can be networked to HELIX 10 to share Waypoints, Accessories, and Sonar ...
2: Yes the HELIX 2d can display SI/DI from the 999 SI xducer ...
3: Networking is advantageous regardless of which new unit you add ...
Some cost can be saved in choosing the HELIX 2d Sonar unit over the HELIX SI unit ...
Disadvantage (in the HELIX 2d Networking) is the 999 must be powered on in order to use the SI/DI from the 999 xducer ...(where a HELIX SI unit could utilize SI/DI/2d without the 999 being powered on) ...
Rickie