While it may seem cut & dried to you... unfortunately it can be read either way as to whether it's a sustained spell, or sustained critter power. I've seen people play it either way... and I also prefer the 'sustained spell' position... but I try to be neutral when answering rules questions.
As for the second... the problem I was pointing at... was what happens when the 'dual entity' goes away. The spell was cast on a valid target dual-entity... When it disappears does the target of the spell disappear with it... Spells don't retarget themselves by the rules... they only affect one target for as long as that target is valid. That's the grey area of the rules I was pointing at. While possessed the mage & spirit become a singular dual entity for all purposes... when the possession ends... that dual entity ends. The dual-entity is the target of the spell. When it ends.. the spell cannot make decisions for itself... it's original target is gone. Does it 'end' because it's original target is gone, or does it linger on the vessel.
And before you answer that, remember, spells are more than only physical. What if it's a mana spell like 'mystic armor'. If you say it lingers... then there are 2 copies on two valid targets now... one on the spirit one on the vessel. Spells don't work like that, they don't 'split' in two. See the issue... and mana spells and physical spells work the same as regards targetting and sustaining. (if a materialization spirit casts increase ref on itself after materializing... then dematerializies... the spell goes away and isn't sustained til the spirit rematerializes... the physical spell doesn't bridge to the astral to go where the spirits astral form does it was cast on it's dual natured physical half).
My own opinion is. Spells are 'dumb' they can't intelligently retarget or 'multiply' themselves. When the possession ends... the original target of the spell disappears. And the spell dissipates as per the rules of magic.
The reason I used a belt buckle, ring, or piece of jewelry as a prepared vessel... is because it's a very good trick commonly used by possession mages. The spirit inhabits the ring.. can use it's powers just not move itself... But nothing stops the spirit in contact with you from casting 'increase reflexes' on you (touch range spell) instead of on itself. There is no 'dual entity' problem as you yourself were never possessed... your ring was... he can cast the spell on you from the ring... then go back to the astral or his metaplane to party while sustaining your spell.
Though, as a house rule... I've commonly seen spirits not be allowed to sustain spells unless they stay within Summoners magic X 100m. (remote service range). This way the spirit can be quickly hunted down and dispatched if need be. Though the rules put no limits on sustaining range.
Another case in point is some spells like 'heal' cause problems here since they specifically don't work on non-living items. You get the dual-entity which starts with damage on it from the more damaged of the vessel/spirit. And damage rules which only cover damage inflicted... but not healing. (healing isn't considered damage). You starting to see why the whole dual-entity problem is such a mess and not well handled by the rules?
Mantis has it right...
p183 spell targetting...
"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...." dual-natured can cast on either plane. A spell is only present on one plane... and since an astral target can only cast spells on astral targets he can only cast mana spells while astral. (spell must have a valid target to be cast). Reiterated p182(spells 101), The rules are very strict about a spell even a mana spell is only ever present in one plane or the other. A mana spell cast on the physical plane... stays on the physical plane and never does anything to purely astral targets. Physical spells can only be cast on the physical plane on physical targets.
The link is a telepathic link... which always exists for purposes of communication between the spirit and it's controller. Also for the purposes of astral tracking... (from the summoner to any of his bound spirits... or from a bound spirit back to it's summoner). And for the purposes of astral