I think I see where you are going with this Dark Knight. I have had success with 800 runners using an endurance based program and a speed based program. Yes, there is always a base phase or period to be sure. But, one of the keys to success is determining which kind of 800 runner you are dealing with. Bob Parks has said that he doesn't want the type of 800 runner who is too slow for the 400 and not strong enought for the 1500. Ideally, you'd want to have someone who is good at both in order to conquer the 800. Seb Coe and Joaquim Cruz come to mind as the closest to brilliant in both, but we are not talking about an elite athlete in this case.
Some people favor the Lydiard approach in which your base period is nearly exclusively devoted to endurance workouts. An approach favored by Seb Coe is a multi tiered program which gives you most all elements of training within a 10 day—> 2 week time frame and quality speed efforts are done year round. I tend to be in the balanced program camp as I think it is the training that is best suited to the demands of the race. The complexion of the quality days will change as time goes on, but the scheme is the same with work done at 5 different race paces during each microcycle. I find this method really helps people get through meets with rounds, but I digress.
As to Mike's questions:
1) I've always found that the off seasons were the best time to work on weaknesses. As an athlete, I actually worked on my speed and special endurance in the summer. I could do that because we didn't have cross country at Kent then, but I would recommend it even if you do run cc as an 800 runner.
2) Minimal mileage could vary. For a 1500/800 runner it's probably going to be higher than for a 400/800 runner. With endurance training, I have found that it takes a bit more mileage each year to elicit the same results, but there is a point of diminishing returns. One thing sticks in my mind is something I once read-do enough mileage for the task. That is to say, don't do mileage just to show big impressive numbers in a training log, but do enough to handle the work you need to do in order to be as fast as you can be over your prime distance(s). It's tough to put hard numbers on it.
3) The benefits should carry over, but I would make sure that you don't overdo the attempts to have the strength carry over. As a sprinter sometimes uses the 4 x 4 to gain special endurance during the indoor season, I would use the cross races as tempo efforts and not be too concerned about training too specifically for cc. I don't think it's cc that diminishes speed so much as it's neglecting speed in your training.
In general, I've always felt that one of the training keys is to be able to handle relatively high intensity with rather minimal recovery.