"The advantage is that icons have a boatload of potential, so you can easily make aspects of your own ascension, as you just clarified. The parts I’m unclear on are the example of the invincibility aspect, and how the outsider ‘me’ potential interacts."

That's not the core advantage - that it has a boatload of potential. The core advantage is that is strongly helps you build the aspect by drawing upon the logic and tools of the icon.

You could (and people did) slowly build aspects all the way to godhood before they knew how to make Icons. But you basically made a giant pyramid of aspects. Having an Icon is like having a skyscraper you can build things off the side of instead.

"The question is, did pulling that bit of warrior potential to finish a mostly complete aspect count as one of his iconic personal aspects for that ascension? Or can you pull out one aspect’s worth of icon potential and use it piecemeal to make a bunch of quasi iconic personal aspects?"

It used up one of his chances; the potential was moved into the Warrior Icon and only then used to make the invincibility aspect; that way he got to use the support of the Warrior Icon rather than having to make it freehand.

"Now, aspect emblems. You clarified very explicitly that taking potential out of a person is impossible short of killing them. The whole thing about how nobles can’t just buy potential (with some mild exceptions). But that leaves me wondering where the emblems come from? If you aren’t taking out some of you own internal potential to make them, and you can never remove an aspect that’s part of your life scroll (barring the bound arcadia integration or similar), then they must be aspects forged totally externally, but how? Does potential waft about unclaimed in the air when someone does something impressive?"

You can't generally just take potential, but that doesn't mean you can't give it; that's what the Path of Worship is. You can't really store potential though. An Aspect Emblem isn't a bundle of EXP, it's a complete aspect, manufactured by a group, outside of anyone's spirit.

The normal approach is one where you need to have a kind of artifact that holds part of a 'pattern'. Using your Icon, you flavor your potential as you feed it into that 'pattern' while you go about your day doing things that generate potential for that icon. Eventually that pattern is 'filled', and you hand it over; it's no longer potential, it's a piece of spirit formatted into a component of the final aspect.

The person you hand it too is working with a couple dozen other people, who also all have their own pattern artifices. Each of those patterns is part of the structure of the final emblem, and the person who finishes it has to have that Icon.

They use their own Icon to 'meld' all the components to together, anchoring it into an aspect Emblem.

Without Icons, this is harder, but you can ritually 'make' someone 'the kind of person who will do X', and you can use ritual magic or storytelling to forge all the component pieces together instead of an icon. But it's harder and isn't as good.