Unstoppable is definitely an option, but I prefer a more balanced defensive setup than relying on one skill to keep me alive. That's actually one of the main differences between cctobias's build and mine--the other main difference being unarmed combat vs. staff+shield.
As far as getting generic talents from escorts, it's a big help. You do have numerous skills that can benefit from extra levels even if you don't unlock any additional generic categories though, so it just comes down to preference. Using Succor mod and unlocking FEM seems like a reasonable option.
Ultimately, you need to decide if you want 5 inscription slots at endgame or if want 4 slots + another category unlock. The build in the OP uses all available category points even without Spell/Temporal. If you wanted to really take it to the limit, you could go with 3 inscription slots and 2 extra category unlocks. It would be very tight on talent points though, and you'd have to make some hard choices on which inscriptions to use.
Just wanted to make it clear that I chose Unstoppable as the main defense very specifically for Madness. I am almost 100% sure you could do Insane quite well with some alternate (something that is essentially complete with two lines). On Insane I can definitely swap out one of the offensive enhancers for a defensive line and swap Unstoppable line. So a two line defense less chessy build is possible there. I specifically chose to make the Meteoric build to be a Marauder+ build. The entire point was to copy how marauder did things since I knew it could win, but make it better. So this meant leveraging Unstoppable. I wasn't trying to win Madness in a new way. I actually specifically copied something known to work (with some caveats i.e. needing things like double mercy build after HP inflation happened). The "strategy" of the Meteoric is in no way innovative or new, its just a build to execute a known strategy in a more OP way and a couple extra good things.
For Madness I needed the absolute most damage I could get combined with the cheapest most effective defense possible. That is definitely Unstoppable. Its as cheap as it can get as far as point investment, both cat and class. But equally and perhaps even more important; it was something proven to work on Madness. Of course the idea of the build is sort of inextricable from Unstoppable because the entire point is to kill absolutely anything (except that damn arcane burn slime boss) and kill a rather large number of things before the time ran out on Unstoppable. But this came about from the initial of idea of "copy a working class: Marauder".
For the most part I don't like using Unstoppable, frankly its uninteresting to me. But for pure optimality its basically impossible to beat. So you can always consider Unstoppable. But I think for Insane and below builds you can use something else and make it work. And personally I think its worthwhile to do so just from a "fun" perspective.
One caveat there is that Unstoppable would suck for something like an Archmage which wears stuff down, you do basically have to kill and move fast for Unstoppable to be worthwhile for anything other than a way to stop a large alpha strike.
In the guide I specifically address where I think there may be some wiggle room, especially for lower difficulties, specifically because of this. But caveat emptor, the Meteoric build was much easier to keep on course exactly because its strategy was known and tested by Marauder. The only thing I really had in mind as far as uncertainty was "Will I do enough damage?". This was actually quite important to narrow down my concerns due to way Madness works out. Not only is it much slower to do, but its also much more failure prone. So I wanted as few questions as possible.