1, 4. I have seen people drop from 3 hour fitness to sub 2:50 fitness during an effective marathon buildup. With London being a fast course, you have the potential for significant improvement. For now, though, you should base your training paces on the current baseline fitness indicated by your upcoming 10k race. If you periodically race or time trial in the 5k-half marathon range, those times will provide a gauge as to what marathon fitness you can expect at London. The basics of your training plan will be identical regardless of where on the 2:50-3:05 spectrum you fall. The paces should evolve over the coming months, but the actual workouts will be the same (provided you begin your buildup with decent fitness and aren't coming off a big hiatus).
As far as structuring your training on a big picture level, I would focus on 10k thru half marathon fitness for the next 6-8 weeks before a transition into a marathon focus. Once in marathon training, the structure becomes quite simple: two Big Workouts per week until about two weeks before the race and then begin a light taper by keeping your mileage relatively steady but cutting back your intensity.
2. Yes, you have the right idea regarding Big Workouts. Note that you can make a Big Workout out of any quality day by adding mileage to the warm up and/or cool down or by adding a second run for the day. Additionally, if you go on a regular easy long run that can also count as one of your Big Workouts for the seven day week. You might then have a two week Big Workout rotation of
Week 1: a) 5 x mile at HMP within 2 hour run; b) easy 2:15 run
Week 2: a) 4miles-3miles-2miles-1mile at marathon pace within two hour run; b) Two hour run including 6 x 3 minutes at CV pace + 4 x 200m at 3k pace
3. I don't see anything terribly wrong with those speed sessions you have posted, but I would ask what you are trying to accomplish during those sessions? The Combo workouts that you are doing (mix of hills, sprints, extended intervals) are perfectly fine, as is the relatively unstructured nature of the workout (which can teach you to read your body in response to different stimuli), but I think you can probably be more efficient with some refinements, and that can begin by defining what you seek to accomplish with the particular workout.
A bread-and-butter speed workout for anything in the 5k-marathon range is what we call CV (Critical Velocity) intervals. Once you get your 10k pace established, you can run about 3-4 miles worth of 800m-1200m reps close to that 10k pace with about 200-400m jog recoveries. After the main set you can add some strides (about 600-800m total) at 3k pace or faster. You can do CV reps every week during early base training, though as the marathon gets closer you might drop to two weeks out of every three and then ultimately once every other week during the peaking phase.
For faster work (3k-5k pace), you can continue with a similar theme to what you have been doing, though within the workout I would work slower paces before faster paces and hills before flat speed. Thus, you might do a fartlek session of 4 x 2min at 10k, 4 x 1 min at 5k, 4 x 30 sec at 3k. For hills, you could do 10 x 100m (short hills) followed by 10 x 100m strides, followed by 10 x 50 sprints.
