So much to unpack here.
First regarding apex=tangent. A tangent on a circle can be be anywhere on the circle. So I'll use Mr J's standard that apex refers to the center of the board. So what GiGi is referring to as apex is acceleration of rocker/angle of attack? (Mr. J is now my official GiGi translator).
I am not sure I always understand Greg, however he definitely uses the convention of apex being the tangent at the centre of the board and has previously said that is where he would place his rocker stick too (something that only needs to be done to de-construct boards now that we have the 3d software). However I think Greg was referring to just lowering the rocker by pushing the apex upwards which would alter the angle of attack - or maybe he meant keep the same overall end/tail rocker but flatten the apex producing more nose/end flip - either way angle of attack is affected although in opposite ways.
Regardless the interesting point is that while one shaper might consider a flat "speed box" in the middle Greg is considering angle of attack Another thing that I have picked up on is that while other shapers might resort to shedding excess lift in the tail via V or V double (for stability), Greg instead keeps it flat but controls it with extra tail rocker and consider how a diagonal path over a flat tail with extra rocker will result in a curved water path in the direction that creates suction as you illlustrated in your inverted air-foil diagram.
And regarding the lift (planing) on the bottom (rocker) of a surfboard, it's the opposite of a foil's lift? It's pushing up against the convex surface instead of pulling on the convex surface? So it's really a totally different animal.
Pulling up on convex surface of a foil is responsible for roughly 50% of the lift. Pushing up on the underneath of a foil from deflecting the fluid down provides the other half of a foil's lift.
So we do have an underneath deflection/flow turning surface on the underneath of our boards, however I have come to believe the underneath of a planing surface, surfboard in particular is a very different animal to the underneath of a foil. From reading some of naval architect Lindsay Lord's book on planing hulls it seems the flow on a planing surface is a lot messier than the underneath of foil. A submerged foil's under surface is much more efficient. With a planing hull we have the ridge of high pressure at the spray root. Only some of the surface is actually in contact with the water (as pointed out by Mr Sopa) and there are regions of suction too.
Here is a quote from Greg that manages to encompass my waffle (I think).
Entry angle helps create initial lift up to plane - angle hand upward out car window to experience this ;-) - No foil involved
Tail rocker controls that planing effect and release of planing pressures , still entry and exit angles all thru the back half depending on the angle the board is riding on .
Stretches 5150 reverse channel...hmmmm... to me looks like water flowing over that will release, create turbulence and make the rail harder to engage. But I'll trust the anecdotal evidence of anyone who has ridden the board.
I wish someone would tape some of the red yarn they use on sails to the bottom of a board and then film under water. I remember someone on Swaylocks did it with a clear piece of plexiglass in a pool just to get an idea (Gdaddy?), and it was interesting. Most of the water went at an angle up (that becomes spray off the rail). But at the deepest part it was angled down. I think the yard would look like asymetric fishbones, if that makes any sense.
I remember MeeCrafty taped streamers onto the underneath of some plexiglass, then put a hose on it and posted some pictures. If someone took it a step further and planed it across a surface of a pool then that is great (I don't remember that, but my memory isn't great). I borrowed Crafty's idea and taped a curly piece of gift wrap underneath my board and noted that it emerged diagonally outwards most of the time. A company called Camber surfboards has taped multiple short streamers all over the underneath of a board with underwater camera and it did show a sort of curved fishbone pattern like you say.