Questions:
1) Can a mage be endowed with possession or materialization by a great form spirit?

2) Can mage of the same tradition summon spirits of the same type, have them take a meta-planner shortcut and pass messages back and forth on the meta-plane virtually assuring privacy? Yes this would burn through services like mad, likely 1-2 per message but is it possible?

1. Functionally no. Since you need to be in Astral Form, rather than merely Astrally Projected to use either. However, combined with another spirit giving you Astral Form, then you could. But you'd have to be projected to begin with.

2. Why would you do that? First step: bind a spirit. Second step: have it go stand somewhere very far away from you. And then it can still communicate telepathically with the binding magician.

You imply that a character can be endowed with more than one power if they come from different spirits. The text "No character may gain more than one power from a spirit in this way at a time" lead me to think otherwise.

Frank wrote the chapter on Spirits in Street Magic.

Note that it says you can't gain more than one Power from A Spirit, and you wouldn't be with Frank's suggestion. You would be gaining two Powers from two Spirits.

...At this rate we're going to break the general SR4 board record for tags. Also, I'm not really sure why you'd need a -real- Spirit to do any of this to begin with, much less a bound Spirit. Watchers share a mental link with their conjurer just like any other Spirit, and they're quite capable of heading to a specific place and manifesting in order to relay messages (the Courier service) on the physical plane, so I would think that if anything it'd be easier for them to do on the Astral, since they are after all purely Astral constructs. I guess you may want to use a more powerful Spirit occasionally in the interest of making absolutely certain the message gets though (Watchers are quite fragile and not so bright), but between the mental link and their low profile, Watchers would be pretty ideal for most situations. I guess a lot depends on if you're busting through a message against opposition or avoiding detection in the first place.

His other regret is losing his token.

You know me so well.

An unbound spirit, of which Watchers are a subset, can only send or receive telepathic communication from their conjurer out to magic rating x 100 meters. A long way in an apartment building or an arcology or even a city. And there are certainly crazy things you can do with that. But if you want to have telepathically secure communications across any real distances, you need a bound spirit.