c) But, as I am receiving NMEA from the depth sounder, could I mix
this info with the NMEA from my VHF radio in the COM1 to receive the
"Position Reports", assuming that my new radio has NMEA in/out?

You can not simply "mix" the outputs of two NMEA talkers - you need to
use an NMEA multiplexer to combine them.

how much do these things cost? I found one once and it was about USD
500