Well, I’ve been thinking about this tonight, and I can answer one of the OP’s questions.
As some of you know, I‘m a grain farmer, so I understand a little bit about the germination and early seedling development of cereals. The malting process starts out encouraging germination by alternatively soaking and draining the water from a batch of grain. During this tIme the grain is closely monitored for the first signs of germination. When the first, tiny, root is apparent, and has reached approximately the length of the kernel, the grain is slowly dried, in a process called kilning. This raises the temperature to about 150° F and removes almost all of the moisture from the grain. This stops the kernels from growing any further, preserving most of the starches to be converted later, during the mashing stage of the brewing process. If the plants continued to grow to the stage pictured above, there would be no enzymatic activity remaining. The starches would have been converted to sugars and amino acids which feed the growing plant until the root system is developed and the coleoptile (which becomes the stem) has emerged and leaves begin to develop, enabling photosynthesis to begin. So, in short, grain intended for brewing is allowed to germinate and grow just enough to indicate that the enzymes, which will later convert the starch to sugars, have been released. Stopping the plant development at that point preserves the maximum amount of convertible starches.
As for why the malt syrup process uses grain which has grown to the point of establishing second and third leaves-I’ve got nothing. At that point, with a developed root system and multiple leaves on a growing plant, there’s not going to be any enzymatic activity. And, using just the new growth, without what remains of the kernel, doesn’t appear to offer anything in the way of converting the rice starches. There has to be more to this process; maybe someone else can fill in the blanks.