I completely understand where you're coming from in the first part, but tbh, as far as the Hybrid tank/caster part is concerned, caster first, tank second, but it runs a close second. I've always had a lot of tanking success with Maokai for some reason, that's why I posted one of his pros as being "inherently tough" regardless of what I have.
I also couldn't stress enough that people could and should reconsider the items, and/or masteries considering the circumstances and their own personal play-style, hell it could just be a matter of getting one of those items before the others if that's what it takes.
The core items are really all I recommended to keep in order to retain the original goal of the build.

Otherwise, my skill combo was as such for some players who aren't 100% sure of their aim for Sapling Toss. The root guarantees the first hit of it, followed by the arcane smash (due to pop up) is going to give the sapling at least an 80% chance for the second hit. Granted, the combo can be interrupted due to a well timed cleanse. Your skill order is perfectly fine, mine is perhaps just a bit more.. "noob to Maokai" friendly.

I love the name of the guide but I find it is lacking. You mention Maokai as a tank but this is most definitely not a tanking build, you have barely any resistances (and you suggest building a Deathcap before additional MR, after Mercury's Treads). You won't be able to tank with this build. Rod of Ages is too expensive early game, which will leave a massive gulf between your tanking ability (resistances) and enemy's damage. Maokai already has very bad early game tanking, because he must solve his mana problems before building tank gear (I cover this with a Doran's Ring and Glacial Shroud).

I'm sure this is a great tanky AP build, but you'd be a terrible tank with it. I'm not going to vote you down, but I think you should clarify this. Anyone looking for a build to rival Malphite or Amumu is going to be sorely disappointed.

You also suggest Twisted Advance and THEN Sapling Toss. I find that if you use Sapling Toss, THEN Twisted Advance you have much more reliable damage and can then skill your Arcane Smash over TA (which you'll need to max if you throw Sapling after your root).