I mean... we have no basis to judge that at all is what I'm saying. Sakurai said each time you go to the island; if it's a static stage, it's definitely on load, but if it's dynamic, there's basically no way it could work other than leaving the island. I'm all for reading into exactly what Sakurai is saying and extrapolating, but in this case, I think his language is incredibly ambiguous to the point that it could be either way. Like... this update is so vague that we don't even know what parts of the stage constitute "ground"... I do notice that it's surrounded by water though; swimming confirmed to return?
As per whether you can specify forms, it has gone both ways in the past on stages with multiple load-outs. Mushroomy Kingdom let the player pick 1-1 or 1-2 based on held buttons, but Spear Pillar and Shadow Moses Island (random background Metal Gears) did not though to be fair it's also not clear just how different these various layouts are. If they're really similar, there may be no reason to let us pick but also not a huge amount of significance in which one is picked.