Question 1: I was wondering if you could be a fair enough assassin by using:

Illusion for stealth.
Conjuration for summoning dagger or bow.
Destruction for setting up fire, frost or lightning traps.

What do you think, yes or no?

Question 2: And is it possible to find Conjuration spell to summon a dagger?
Please just answer yes or no, NO SPOILERS please!

Of course it's possible..you can do anything you want

I wouldn't use Destruction myself because they don't get any stealth multipliers. But if you are conjuring weapon like a bow make sure you work on Archery because your Conjuration skill doesn't increase your bound bows damage. Definately go Illusion though!

Definitely is possible.

Thing the you want to avoid until you get REALLY high in stealth is run of the mill spells like fireball or lightning. I forget if it was the manual or in game that mentions that casting a spell from sneak mode in the darkness basically lights up your location and is not advisable. The skills you listed though I could see working VERY well.

Finding the summon weapon spells may be difficult (not sure since I'm not a mage), but daggers weigh so little you might as well bring one until you get the corresponding spell.

Good luck. Sounds like you have a good idea.

You can be whatever you want to be! Follow your dreams!