I agree that putting the core closer to the engine may increase the potential for heat. And I'm sure it's less expensive for the hose, and easier to run it.
I've done several heater installs & never understood why it was buried under the driver's side dash, behind the subwoofer. I usually put it under the passenger side dash. But now days boats have a ton of stereo gear under there that could be difficult to work around, and damaged if/when the core ever leaked.
The Hot Tubes are the most used vents in most any heater. I think when I do another install, I won't bother with a regular vent & just do all Hot Tubes.
As for winterizing, a couple of suggestions. Make sure the core is installed so that the hose inlet & outlet are on the bottom side. And install either quick release hose fittings (like for a garden hose) or Prestone Flush Kit T fittings in the hoses down in the bilge area. I have always done the Flush Kit fittings, just pop the caps off & the heater drains into the bilge in seconds. I winterize my boat like a dozen times a year & never have a problem doing it this way.
And last, check into getting a couple of Hot Wrap boat blankets
. We use these far more than the heater. And the two working together will satisfy most anyone.