So I'm just Wondering if theres any way, using the CS, to make summoned creatures last until they are killed in combat?

Just a thought--you might want to check the script that's used to generate an Atronach Familiar in Frostcrag Spire, as they last forever.
Alternatively, you could just set your summoned creater to last a REALLY long time - like 9999 seconds.

Or make the spell simply do a 'MoveTo' with your creature of choice.

Upon death, you could then fade them out and move them back to a 'storage' location. And then Resurrect it, ready for the next summoning.

My http://www.tesnexus.com/downloads/file.php?id=16308 mod does this with the summonable horse. Except, its a summon to me, then summon back home on the horse. But it would be easy to change.

Or, you could just do a 'PlaceAtMe' with your creature and do nothing else with it. If it dies, it will lie where it fell until cleaned up by the game engine later.

I created a Wiki on this ages ago... Perhaps it could help in your situation? Of course, my situation required a static object to summon the creature, but if you have the spell script, I'm sure it'd be a simple tweak. You can find the tutorial here:
http://cs.elderscrolls.com/constwiki/index.php/How_to_make_a_Static_Object_Summon_Creatures

That was a really helpful tutorial, and I got it to work (in a simple test cell, not a real mod, as I'm just learning). There was just one problem: I couldn't see any way to make the moved creature behave as if summoned -- that is, to be loyal to the player. What did I miss? Probably something obvious, I'm sure! Thanks for the tutorial and any further advice.