Hmmm, it doesn't look like a low-angle bed to me. I've just spent 15 minutes trying to find out what the bed angle of a 75 is, but all sources I consulted remain stolidly silent on that point. Could someone with a 75 & a protractor of some kind measure the bed angle? It would be a very handy parameter on which to base any rational advice on which way the blade should be.
Roberto, looking at your first lot of pics, it seems to me you have a pretty steep bevel on that blade, so clearance may have been the issue when you had it bevel-down. A good rule of thumb is to have at least 10 degrees of clearance. If the bed is 45 degrees and your sharpening bevel ends up at 30, you have 15* of clearance, which is adequate, but if the bed is x* lower than 45, the clearance angle will be less by that amount.
In your pic with he blade bevel up, it just doesn't look right - if your bed is 45 and your sharpening angle is ~30 you now have a cutting angle of 70*. That is not very practical on a larger plane, the cutting resistance would be excessive. On a little thing like the 75, just knocking corners off, cutting resistance won't be such an issue.
Lastly, bull-nosed planes are always tricky things to use. They are very sensitive to pressure over the nose, you can alter depth of cut quite markedly by pressing on the front of the plane. They are also very awkward to start cleanly from an edge, you just don't have enough surface area on the toe to register it & begin the cut properly. I would suggest any small plane with a toe 1/4-1/3rd of the overall sole length would be much better for your stated purpose than the 75 or any other bull-nosed plane - the Stanley 100 or any of it's multitude of clones make very handy one-handers, ideal for knocking sharp corners off....
Cheers,
Ian