This skill is wellthought and is a very good read. Although i see one major problem, which is the involving of all of the other skills. Not that the skill include some requirements from the other skills in the game, but that the skill relies on these, this won't make it a skill itself, but some kind of mini game, or tasks. However it is very interesting, and you have covered most of what is imagineable. Well done.
It is a great idea to make some extremes before you reach 99 herblore as already mentioned, therefore you should consider making these when you reach 89 and stop at 91. Now you will have at least a stock of extremes, that you could safe up for making overlaods at a later point, or simply use them for pvm'ing. A note would be it is considerebly better to use extreme potions on example slayer tasks, unless you're very wealthy. At the same point they're quite good experience. If anything should frighten you to not making extremes for this weekend, it would be that you would never gonna use these mentioned potions in combat.