Not too much to add here as most of the ideas are spot on. lR has evolved this year, though we still see the editorial bias in headlines such as those mentioning Teter and her strength work. My thoughts were oh, you mean the strength work which ended her racing career? Vessey with comparable 400m speed ran fast or faster on half the work.
Anyway, I wouldn't let a 50" 400 runner consider the 800 unless the goal was <155 at least. I consider <50 relay speed more or less the basic requirement to winning any HS state meet 800m (at least in states with more people than cows).
400/800 need goals at 400 and 800m and to see these goals as closely connected and progressing together -- at least for them. Figure roughly 50% the volume of a 800/1500 runner at that level.
Absolutely no reason to sacrifice speed (or fun)for stamina gains. I like to use the faster running to reinforce the stamina work.