Well, I think your answer is "no", because you aren't actually in the physical world at the point of your manifestation.
Manifesting is a psychic effect that allows an astral form to make itself visible and audible on the physical plane through an act of will.
...Unlike the Materialization power of spirits (p. 296), manifesting does not create a physical form, and so the character cannot physically interact with anything, nor can she be harmed by physical attacks.
So you're still an astral form, active on the astral plane, just visible/audible on the physical plane, but not really there.
A magician in the physical world can only cast spells on targets that are in the physical world. Similarly, a magician in astral space can only cast spells on targets that have an astral form *though the auras of things in the physical world can be seen, auras alone cannot be targeted). An astrally perceiving (or otherwise dual-natured) magician can cast spells on a target in either the physical world or in astral space.
Manifesting doesn't make you dual-natured, nor does astral projection (it severs the link between magician body and magician astral form, allowing said astral form to roam away from the body).
EDITTo answer the implied question: "Can Merlin the Magician cast a limited spell while astrally projecting?"
Yes, if the fetish is touching his physical body when he tries to cast the spell (SR4A, pg. 182). This is the only requirement, though I would consider house-ruling that any fetishes on the mage when he astrally projects appear on his astral form & he needs these astral copies on his astral form to cast the spells. Seems to fit the flavor of astral projection better, given that you sever the connection between mind and body (I know, it's not completely severed, since damage to one affects the other, etc).
