1) If you have a heater on the port side...it will fill your entire storage port side.
2) Starboard side is the same length...but no heater gives you about 18-24" of unfilled space under that seat. (i kept a bit of lead and my tools there)
3) 2019 Mojo should come with the following upfront:
a. 700 lbs in floor upfront
b. 500 lbs IBS under seat

The Exactfits made a big difference over the stock 900 rears. I wanted to install some mesh cargo nets above my rear bags for life jackets and wet accessories as you probably get about 8" space above bags in rear lockers.

I found that i rarely needed more weight upfront if all 1200 lbs was full. If anything I would have to drain some of the front bags to get optimal pitch on my 2019.