Though I appreciate your logic, I’m not sure you have the problem with edge jointing analyzed correctly as due to a concave sole. Rest the plane on a flat surface like a table saw table. Can you see the concavity? My planes have been carefully flattened, and it’s still easy to wind up with a high spot in the middle when joining boards, which probably has a lot to do with the pressure one is exerting. It’ easy to get more pressure at the beginning and end of a board. It can also be difficult to not weight the plane to one side or the other which also screws up the jointing. And grains can affect the whole process, leading to high spots close to where knots were, etc.
Check out Paul’s video on edge joining two boards: https://www.youtube.com/watch?v=h-vMtS_j4cY This is the technique I use to edge joint and it works great. But I’ll add that after a first try I often have a high spot in the middle of the two boards. I then put the two boards back in the vice, take a couple of passes to make sure the boards are back in alignment, and start making a couple of passes just over the center few inches. Then I gradually increase the length of the passes (maybe a total of 4 or 5 times) until I’m almost at a full pass, and then do one final full pass. I’m doing all this with the plane adjusted to take a very, very light shavings. I’ve also found it important to make sure the plane blade is adjusted to be dead on 90 degrees to the sole. If you’re not familiar with how to do this Paul has excellent videos on adjusting the blade with the lever, and testing it by taking shavings from both edges of the plane. You listen for the sound to tell if the shavings are equal in thickness.
Also, I get the board edges individually to 90 degrees to the face using a shooting board before doing the edge jointing, so I’m just fine tuning in that step. I know it’s not necessary (as you’ll see in the video), but it works for me and makes me more confident that the edge jointing will work out. And good luck Roberto – I’m sure you’ll get this problem figured out.