I haven't been naming the Loa since they haven't had significant roles, but since this is a recurring summon, and it's about to have a significant part to play, I guess I should. Of course, that brings me to the real reason I haven't been naming them - it's complicated. The Loa belong to families, nations, pantheons, and what have you. I haven't quite wrapped my head around it, but we'll give it a shot.
I found a list of named loa
here. I'm not sure if you really summon these specifically or if they are the heads of a court of similar, lesser, loa that are summoned. I'm assuming the latter, but I also assume that the lesser loa are unnamed, and for simplicity will refer to them using their master's name. So the sneaky loa that loves the swamps that I have summoned will be Gran Boa. Note that these are Haitian loa, though everything I have been able to find on Louisiana Voudun seems to indicate that the loa names are largely similar, having come from the Fon and Ewe slaves that carried the beliefs from Africa to both areas.
You are correct in the actions it will attempt - however, if it is undiscovered, it will simply crack the sliding door and place the ring on the balcony where it can be snatched easily, only going fully outside if necessary to maintain its cover.
While possessed, the spirit's skills and mental attributes are used, but the host's physical attributes are used. However, if the host's physical attribute is less than the spirit's force (7 here), half of the spirit's force (rounded down) is added to the attribute. It's unlikely that Skim has an agility higher than 7 (not sure if this comparison includes augmentations, but I assume so - at least the ones the spirit can benefit from), so it'll probably end up being Skim's attribute + 3 (for 7/2 rounded down). So, the stealth check will likely be Skim's Agility + 10 (7 for the spirit's sneaking and 3 for the possession bonus). Also, if Skim is inebriated or otherwise negatively impacted by a stun or physical damage effect, the spirit reduces the Wound Modifier by 7 (one point per point of Force). This applies to damage taken both before and during possession, though any damage taken during possession is also applied to the spirit, and it does not negate that. In short, 7 points of WM negation (7 dice, not 7 boxes - basically the equivalent of 21 boxes of damage negation) likely means that Skim will be into the Overflow box before his wounds affect dice pools.
So, summary:
Sneak, run, unarmed: Skim's Agility + 10
eliminate up to 7 dice of WM due to damage Skim has taken - stun and physical
Can access any augmentations that Skim might have that do not require DNI (the rules say "active control", which might also include things such as an adrenaline pump or pain editor). So, bioware can be used, but wired reflexes could not.
Skim is unaware of what is happening while possessed, but the loa also does not have access to Skim's knowledge, skills, or memories.