We had the Interphase unit on Elyse when we bought her in 2013. About a year later the transducer failed. I obtained a replacement from Garmin in the US who had just bought Interphase. That transducer failed also, so I took it all out and got a 7" B&G plotter to go where the Interphase head unit was on the port side of the helm, and installed a ForwardScan transducer for it.
The ForwardScan system works OK in smooth water and at low speed, but if there's any slight pitching of the bow, it can't handle it and the picture goes crazy.
The more expensive forward sonars (eg Furuno) have sensors in them to compensate for boat movement and keep the picture steady. Not sure if Garmin has that.
As to hooking something up to a Furuno Navnet radar plotter, I doubt you'd find anything to do that.
BTW I have 2 working Interphase head units on Elyse, if anybody wants them, you can have them for the cost of the postage, whenever I can get back to the boat.