I have a 1992 20' Tiderunner is the daddy of yours that probably uses the same system. There may be some mis-conception as to the term "Self-Bailing" on both your side & the dealer.

Mine has plugs in the deck holes that I keep in until I need to drain fish blood or wash the deck down. These plugs are smaller than normal drain plugs, so I just buy regular drain plugs & grind the rubber down to the right dia. on my 6" grinder. Now you can push them in & snap the lever over to tighten them in the deck drain. I run them plugged like this 98% of the time & only take them out when I need to wash down the deck.

Yes, with the plugs out & a heavy person standing in one corner, water will come back in thru the overboard drain. If you stand there long enough it will run under the battery hatch cover & into the bilge. The rubbber flapper valves on the outside of the outlets on the stern do not seal well enough. I bought from West Marine #3684107 which is a plastic hinged lid type scupper that replaces the originals. This is a way more secure unit what came with the boat.

I just went out & measured as best I can & the deck drain is about 9 1/2" above the center of the scupper outlet. As I see it with these boats being a DEEP VEE they appear to be more tippy with weight on one side than a flat bottom boat. But you would loose seaworthyness on a flatter bottom. You will never find one boat that will be everything.