My brother is diabetic. Generally, he’s able to eat pretty much what he wants (within reason - he generally avoids cotton candy binges and the like) - the other side of that is simply figuring out beforehand to what degree that meal will affect his blood sugar, and adjusting his insulin shot to accommodate. Since that can’t really be a perfect science, he also checks his blood sugar regularly.
Something I’ve noticed as time went on from his initial diagnosis years ago is that as he became more proficient at estimating the glucose effect of his foods, he was more able to just sort of “wing it”, make some food, have a general idea of the effect it would have, and not spend as much time micromanaging his meals. The takeaway with something like Soylent would be the ability to know exactly what is in an X-sized glass, and any time that’s the meal, just knowing to use Y amount of insulin. Having a “default meal” also means having a default, easy-to-remember insulin dose, which could be pretty useful.
Also, to answer
Sort of - it kind of varies per person and degree of diabetes. Basically, diabetes means the body doesn’t quite react to blood sugar changes appropriately - which can go either way. The pancreas could make too much insulin, or not enough, either of which is unpleasant (or at extremes, dangerous). Most diabetics have to take insulin shots along with meals just as a regular thing (and conversely, have some trouble if they go hungry, or miss a meal after taking a shot in preparation). How much the pancreas misses its mark by is what differs - it could be just a minor thing that pops up occasionally, or it could be pretty terrible at its job, and require keeping a solid eye on blood sugar/etc.