I'm surprised no-one has bothered to tackle this.
Sorry it's me who's going to start, because it would be here, unprofessional, player opinion...
Firstly, yes to as many MPs you can dump into it, rather than having to learn higher levels of the spell. I base this on - relative uselessness of a 1-pt spell; and the wording.
Secondly, it would make sense that the Variable option allows for multiple entities... But that's only in consideration of the above making the Variable tag mean something else. (As well as that Summon Spirit Dog 4 thing from somewhere)
Active... I'm uncertain. It makes sense that it needs concentration for the spell itself to finally summon the entity to where you are (I'd suggest non-instantaneous), but after it arrives it stays (visible??) for the regular 2 minutes of a Spirit Magic spell. And thus, no.concentration required. (Obviously, others may suggest that you need to concentrate to keep the spirit local and not have *the option* of leaving.... Which responds to your bit about dropping concentration as a way of removing hostile spirits)
Dispel... Perhaps the spirit is forced back to the spirit plane? Perhaps it's useless altogether once the spirit arrives? (Given we have Dismiss Elemental, the latter is plausible).
I can't see any reason you couldn't get into spirit combat with it, especially if it's hostile. If it's not hostile, you'll have a problem if you can't Control it, or have Spiritspeech... What gestures do spirits consider rude enough to want to attack you after being neutral or friendly??
If we assume that the spirit is temporarily tied to the summoning location, and doesn't become visible, then the summoner is going to need some way to see/perceive it.
I miss the Ritual skill....