Hey fisky, the way I see it (towing 50 pounds for under 10 seconds at a time with 90 seconds recovery) is what I would consider a strength endurance type workout. I think it is a good substitute for weight lifting or short + steep hill sprints
a) if it is hard then I don't think you will get a good track workout in afterwards. If you are looking to prime your cns for a track workout I suggest using a much lighter weight (less than 10% bodyweight) and doing a few starts/accelerations
b) after a track session would be preferred in my mind because this way it won't hinder your track session
c) separate day for strength endurance work is fine, you can probably do it the day after a speed day
d) of course you can mix a heavy sled day with max velocity, but I like to keep them separate. I would keep the fly to 30m or under, 50m fly is hard to maintain unless you are running at a sub max speed with extremely long recovery
The way I see it you can perform heavy sled drags many times per week because you are not moving that fast and taxing your hamstrings/sprint muscles.
I have always preferred light sled pulls (10% bodyweight or less) to work on my acceleration mechanics rather than as a conditioning tool
I sense that you are questioning this workout because you are unable to maintain form. I am in the same school of thought. I recommend dropping the weight to work on acceleration mechanics or preforming steep hill sprints for conditioning