I'll answer some of it.
You can run the unit in Simulator with only the power cord connected to do unit updates, waypoint management, practice with the unit, and play back recordings--view screen shots.
I do all the above at my desk at home.
Yes, you can have the 958 share the 83/200 kHz of either HDSI transducer and either GPS antenna on the network. You select the 2D source and GPS source in the Ethernet menu.
The Navionics+ card is a smaller memory card than the Platinum and Premium editions so the unit won't have any issues reading it no matter what software version you have.
The current Hotmaps Platinum card is 16 GB, your 958, 1198, and 898 are Platinum compatable.
The only recent history issues with Navionics cards is those wanting to renew the Freshest Data subscription and get a year of updates again, didn't have a Premium card large to handle all the new data so Navionics would replace their card with a higher capacity one. The same if they wanted to change to the Navionics+ and their card was not at least 2 GB.
Greg can take care of your software version request.