All spirits and astral beings have immunity to normal weapons. Doesn't matter if they are tradition spirits with materialization or possession.
No they don't. Where do you get that from?
I mean, it'd be pretty hard hitting them with normal weapons if they're only on the astral. But that's because they're on a different plane, not because they have Immunity to Natural Weapons, because they don't - only materializing spirits have that (and only when they're materialized).
Mental & Special Attributes
Possession says that you use the Spirit's Mental & Special Attributes and there is nothing in the exceptions listed under Channeling to change it, but it seems weird to me that you would use the Spirit's Mental & Special Attributes. Am I reading this the right way?
It is weird, but it is RAW. Remember though, that summoned spirits don't use their own Edge, so you can't summon a Force 10 to get 10 edge (page304.SR5Core).
If you have the spirit's Edge, you have the spirit's Edge. And it does have that. You can't make it spend Edge points, but you
can spend your own - and then use your (current) Edge rating to determine it's effects.
But yes, technically it's RAW. It's pretty much certainly an oversight - normally, when Possessing, the spirit has sole control over the body, which is why you use it's mental powers. But that's not what it actually says - what it says is that you use the spirit's stats, period. (I'm pretty sure my GM will houserule this in the same second he agrees that it is RAW).
According to the rules, you resist drain with the lowest mental attribute you or the spirit has.
No you don't, that only applies to Mana spells and powers.
Use combined attributes to calculate initiative. You use the spirits dice (2D6) so not great if you have wired reflexes or adept powers boosting reflexes.
Why? Initiative dice isn't a special attribute.
[Edit] Oh, because it says so later down the paragraph. Okay, yes.