1. Can a spirit sustain a spell that is not of it's type (based on tradition).
2. Does the force of the spell play into the spirit sustaining it?
3. Does a spirit have to materialize to help the spellcaster (By sustaining or providing a die bonus?)
4. Could a Ally spirit be used to basically permanently sustain a spell (Unlimited actions x Force Combat turns = nonstop).
5. If so does this make it better than a level 4 sustaining foci
You already have 2 different answers, so I'll toss mine out there too & give you some pages to reference...
1. Yes. Per CRB Pg. 302 below, you can see that the same Type/Spell is for AID tasks, not for Sustaining which is listed later.
BOUND SPIRIT SERVICES
Aid Alchemy, Sorcery, and Study: As a service, the spirit can add its Force as a dice pool bonus to your Alchemy, Spellcasting, Ritual Spellcasting (for spell rituals), and Learning Tests if its type matches the spell’s category, as listed under your tradition (p. 279).
Spell sustaining: You can have a spirit sustain a spell for you after you cast it. It takes the –2 dice pool penalty per sustained spell, instead of you. It can’t do this forever— only for its Force in Combat Turns for each service you spend on spell sustaining.
2. No
3. Yes, in most cases. AFAIK, the only time he wouldn't is if the Caster, Target, & Spell (Mana) were all in astral space. See "Powers" Pg. 394.
POWERS
In order for a critter to use a power against a target, they have to be in the same state, either astral or physical.
Astral forms cannot affect physical targets, and physical forms cannot affect astral targets (see The Astral World, p. 312).
An astral critter that can materialize can affect physical targets if they do so, however, and dual-natured critters can interact with the astral plane as
easily as the physical one.
4. In theory, yes. But he has to be materialized & will constantly be penalized. Like having a butler walk around wiping your chin after every time you drink. Its going to get old for the spirit & he'll be annoyed. Also the spirit can be targeted by anyone so count on getting shot at. Not to mention the odd looks & down right hostility that you might get from everyone on the street (especially enforcement types).
5. A foci is much better IMHO. You can turn it on/off quickly & its heck of a lot easier to use than summoning an ally spirit just to sustain a spell.