Fair enough. (and irritating, because I just bought that BBB a couple weeks ago)

No, all manifesting does is not allow the spirit to physically interact with the physical plane. I'm not punching him; I'm lightningbolting him.

(Emphasis added) This implies that non-physical actions can be taken, such as casting spells. Otherwise, why would the first sentence even have been written?

Because the astrally projecting magician wants to be seen by his mundane teammates. If technological devices like cameras can't see him and he has no physical form, then he can't interact (read: physically touch or move anything) on the physical plane. He can't even cast spells on the physical plane at that point because he is wholly on the astral plane and manifestation is just a psychic projection of himself for mundanes to see. Think it's a projection on the physical of his projection on the astral. Read the FAQ and errata on the official website and it'll become a bit more clear.

Okay... scenario time... what would stop a projecting mage from heading off to the Metaplanes to avoid dying after their body dies / is killed?

We already know that time doesn't follow normal rules while on the Metaplanes.

That's a good question, but I haven't read Street Magic all of the way through and it very well could be answered in that.

I'm going to break this thread up into two new ones since I still have two questions that are largely unanswered. Thanks for everyone's input, but I am going to try to get a developer's attention to get a flat answer.

Not being initiated.

I'm not talking about PHYSICALLY doing things. Casting a spell uses mana (IE: not physical). And what I posted was the errata. And the FAQ does not cover astral projecting.

In short, there's nothing stoping a Manifesting astrally projecting mage from casting a spell.

The point is that spells can only affect targets that are active on the same plane as the magician. A manifesting mage only psychicly appears on the physical plane. He is still only active on the astral plane IMHO. So he could not cast spell on the physical plane.

Indeed, but not at a physical target, since the mage is not present on the physical plane. When a mage manifests they are simply making their astral form visible on the material plane. They do not perceive the real world. The rules are pretty clear on that.

If you read it another way, you make mages absurdly powerful, since they can float around, untouchable by mundane opposition while they throw fireballs at them.

Right, he can cast a spell, but he can only affect those on the same plane. Manifesting does not make the mage active on the physical plane, it only makes him appear there. He is still wholly in the astral plane. He can still cast and affect astral forms on the astral plane, but he can't cast and affect things only on the physical plane.

Here's and example:

Mage left his body at home. He is projecting on the astral. He floats into a van with his team, but only one is awakened, the other 2 members are mundane. The other awakened individual decides to out of respect astrally perceive but stay in his body so he becomes dual natured. The projecting magician sees the astral form of his one awakened and now dual-natured teammate and the auras of his two mundane team mates sitting there. To allow them to see him, he manifests in the van. The mundanes can now see him appearing as a ghostly image, but he still only sees their auras and the astral form of the dual natured teammate because he still sees on the astral. When manifesting, you never shift your perception over to the physical.