We're on the same page on all of the above, with one exception:
My issue is that during slow seasons, you don't necessarily pass "many, many, many" guests in the standby line. Sometimes, you pass very few.
Take my ToT example. We pulled fastpasses at around 10:30 am and our return time was around 11:45 am. We returned at the beginning of our window. At that point, the standby line was posted at 20 minutes, but there were only a few people in the standby line before the preshow (the line didn't even go out the door of the hotel). After the 5-minute preshow, we still had a 15 minute wait in the boilerroom. We didn't actually board until 12:05 pm.
We were a little miffed that we had pulled fastpasses and waited an hour and fifteen minutes for our fastpass window to open up, only to wait 15 more minutes in line (avoiding less than 5 minutes of waiting). This has happened to us numerous times on our late January trips. They could've made our return time 12:00 pm instead of 11:45 am, leaving enough time for just the preshow and some ushering through the corral.
During slow times, they could make fastpass (and fastpass+) more appealing if they modified the flow of people through the merge point, so those with FP could essentially "walk-on" after their window opens up. This would benefit both WDW and guests.