Steward: Name does sound appropriate, but lacks a certain... spark? Idk, when I hear 'Steward' it doesn't strike me as particularly interesting. I could be the only one though, so, y'know, just thought I'd mention it. As far as the skills go, I like 'em, except maybe for Defender. Kinda funky, and puts the Steward in a tank role, which imo sort of moves away from the idea that he's all about preparation and maintenance. Maybe you could have a skill that grants your entire party a bonus to weapon damage or something for the first, like, 20 seconds of combat? Y'know, because you're prepared. Unyeilding is also kinda in that category, but I suppose some survivability never hurt anyone.
Also I'd suggest renaming Flask of Enchantment to something like Alchemical Catalyst or something like that. A liquid to boost the properties of other liquids. Would makea bit more sense. Right now I get the impression he can enchant flasks.
Apostate: I see you've been pushing the idea of a mage-rogue hybrid for a while now and it didn't really fit the context of the previous times, but now I feel it can work. Magical Weapons and its upgrade could do with more creative names. Resist the Templars sound OP - imo just give the Apostate like 50% to resist anti-magic debuffs or smthng. Energy Boost should be accessible earlier in the tree (maybe replace Resist the Templars with it).
Not sure what to think of this class. There were opportunities to have some more interesting skills, imo, like a passive that added a portion of your Magic attribute to your base damage, for one (y'know, since it's a Rogue). The idea of a Mage hiding their magic is a compelling one, to be sure, and have that character access their magic through a specialisation is a neat way to do it, imo. But, yeah, you could have more creative skills there. I would drop Magical Enhancement for something different, Enhanced Cloak seems a bit iffy (stealth in DAI is already full invisibility, so you'll have to propose something else than just crit increase, since it's now a magical stealth), Arcane Flask is... eh - perhaps make it a passive that tacks on a secondary effect to any grenades he uses? Eg: if the Apostate tosses an Antivan Fire, the grenade will have a secondary effect of weakening armour. Yknow, that kind of thing.
Finally, I know he/she is technically an Apostate, but there has to be a better name out there. Something with a 'roguish' connotation to it, like 'Shadow Dancer'. something that sounds stealthy and magical at the same time (or just mysterious).
Shapeshifter: Ok, now that's what I'm talking about! Some pretty interesting skills in there, and I shall get to it in the next paragraph. First however, you should consider rearranging the tree to be allow for more freedom of choice. Having to pick a steath form, then a shapeshifter buff, THEN your more powerful forms, means your shapeshifter is stuck in leveling limbo for at least 2 levels, which doesn't make this class fun at the start. Consider giving at least one offense-oriented shapeshift form alongside the Mouse, so that the mage can choses whether he wants to start off his spec as a damage dealer, or having more survivability and mobility.
I love those skills. Especially the spider webbing that can catch fire. The Bronto Charge is also a nice way of giving a Shapeshifter a displacement skill while remaining thematically relevant. All in all, those skills sound fun, but keep in mind that if you rearrange the skills to give, say, wolf form at tier 1 (or, really, you can just have a special tree that just has spider, wolf and mouse at tier 1, and bear and bronto at another tier), then you gain access to some upgrades. Maybe some unlock some skills too? The 1/5th chance to heal on reverting back to human form, I'm not a fan of, so maybe reintroduce master Shapeshifter from Origins that unlock more skills for your forms. You could have something like 2 skills at first, then 2 more when you upgrade that form, and then 2 more when you learn Master Shapeshifter,
Also, just as a side note, 250 HP is extremely low in DAI. Consider buffing the health values.