I got a reply back from Navionics about the map compatibility quetsion:

"Navionics charts are not compatible with other software. If your Lowrance LCX-17M has two slots for MMC cards, you can have both your Lowrance and Navionics charts in your unit at the same time. However, the chart information from both charts would not overlap each other. To switch between your Navionics chart and your Lowrance chart, you would choose from the different files listed under Map Choice."

That makes sense to me :)