Bass lakes up north are dependent upon BG for forage for a reason. In Northern lakes, there are very few (if any) largemouth bass that are able to eat GS once they are older than 1 year old - they grow too durn fast and get too large. So, in GS heavy lakes, the available food for LMB is limited depending upon the GS spawn for that year only. Down south, where TFS survive, a larger portion of the forage base can be TFS. But, if those lakes experience cold weather, and the TFS die out, they have the same problems that you are seeing.
Also, as you have noted, GS inhabit more open water, while LMB tend to stay close to cover. Just like you going out to eat, how often do you drive a long distance to eat vs. driving a short distance? Same with the LMB. They will put on more weight and feed more frequently when there are numerous forage fish close to where they frequent. The further a LMB has to swim for it's food, the more calories it has to expend. Then it has to eat more just to make up for those expended calories, let alone pack on pounds. LMB are ambush predators and it's hard to ambush prey in open water.
High populations of GS also take food away from the BG YOY population, making the problem worse and worse.
In southern lakes that have a large population of large LMB (over 10 pounds), GS are a good choice for forage, but not in Northern lakes.
In answer to question #4, most definitely. LMB don't grow as fast as GS, so while you may have a great spawn this year of GS, the LMB can't utilize all the food. If the goals are to grow large LMB, it's better to have a more stable forage base like BG and not a cyclical forage base.