Any ongoing spell effect absorbs some of your Mana. So if you use three successes to summon a shadowy beast, it can be imbued with up to three Mana. The Mana commitment determines how powerful the ongoing spell is. This prevents summon-spamming (you reduce your dicepool for future spells) and helps balance different kinds of spell; it also gives a guideline for how useful things like growing skeletal claws might be.
Borrowing from Numenera, which I've been listening to, let's stick to Might, Speed and Intellect for non-spell activities. You have 11 points to allocate between these, because each needs at least one die.
So, for example: you might cast a Skeletal Claws spell (Assume + Bone) to make yourself more formidable in combat, granting you additional dice equal to the successes you roll. However, while you retain the claws, your dicepool is reduced. If you roll 3 successes, you can choose to allocate up to 3 of them; if you pick all 3, your Mana is reduced by that amount, leaving you only 2 dice.
The claws don't necessarily just boost your combat ability. They might help with climbing, breaking through doors, intimidating people, and so on.
I think for simplicity, I'll use these pools as hit points. Bad things happening to you deplete your pools - note, though, that I'm not envisioning any Mana-injuring effects, so while you might get beaten up, drugged and headachey, you should always be able to try and cast spells.
There's no critical rules for non-magical attributes. Either you succeed or you don't.
NPCs can be whipped up easily. If I want them to have magic or equivalent powers, I can easily either use the existing necromancy pools, or come up with alternative spheres for them, if I want an elementalist or druid or something.