On summoning Stirges:
Thats a very clever and quite valid tactic. Stirges do have some limitations though.
I would agree that 'open space' generally means 'unoccupied square,' although I'd also think you could probably summon creatures into spaces occupied by sufficiently large or small creatures. Generally, creatures that are 3 sizes larger or smaller than the creature occupying a space can move through the occupied space. So I'd think that is probably a sufficiently 'open' space to allow a summons. (i.e. summoning a small dire rat in the space occupied by a huge bullette, or vice versa) I don't think thats RAW though, but it seems a valid interpretation.
(Not sure if you could summon a creature into a space into which it has to squeeze to occupy, such as summoning a large Lion in a 5ft corridor. I'd probably allow it, although said Lion will be subject to the squeezing penalties of -4 to attacks and -4 AC. I'd think summoning a huge Bullette into a 5ft corridor would be right out though.)
Stirges are tiny and four of them can occupy a single 5' space. They also have a reach of 0 feet, meaning they have to enter a targets space to attack. Thus, they will usually provoke an AoO by moving from a space adjacent to a target into its space (for moving out of a threatened square). A creature with at least 5' reach and combat reflexes poses a significant defense against multiple Stirge attacks.
Another strong defense against stirges is any type of 'damage shield' such as the Fire Shield spell, or any similar natural ability such as a Remorhaz Heat ability. I'd think that since the stirge's blood drain takes place if it ends its turn attached to a target, if the stirge takes enough damage from say a Fire Shield spell to kill it it wouldn't be around long enough to do its Con damage, since its dead before the end of its turn. Thus, using stirges against say a Remorhaz would prove useless as the stirges would be vaporized instantly upon attaching--the Remorhaz heat ability does a minimum 8 damage against the stirge's 5 hitpoints.
Since stirges are tiny, up to four of them can occupy a single 5' square. I'd probably say that no more than four of them can attach to a single medium creature at any given time--theres just no space left for any more to get in there. Arguably, one could say that no more than two stirges could attack a small creature; given the size of a small creature vs. the size of a stirge, there just being no more room on the body of a small creature for more than 2 stirges at a time. But again I wouldn't think thats RAW but a valid interpretation.
Given the above, I'd think that summoning Stirges against Large or larger creatures would be a solid tactic. Arguably the stirges could be summoned into the space of the Large creature, since there is a size difference of 3, thus avoiding the AoOs that would normally be due.
A druid won an encounter with a single spell? How surprising...
Except it isn't really winning the encounter with a single spell. Without having the rest of the party distract the enemy, he probably could have killed the stirges by himself at the cost of a few Con points (unless the enemy is a complete feeb in the first place).