Ok I think I am qualified to answer this one, I have 5 cats and 6 corn snakes.
I don't think it's really a stress issue for 4 out of 6 of mine unless it's eating time. All my snakes will go nose to nose with my cats and the snakes seem just as intreasted in the cats as vice versa (Hum, would you make a good meal?) *eg*. But honestly, my cats have never shown agressivness towards the snakes and if the snakes don't want the cat that close, they usually just back off. Most of the time they are giving eachother little "love kisses" (going nose to nose).
My cats all gather round when it's time to feed the snakes. 4 of my snakes could care less if they are watching, two others won't eat with the cats around and will spit it out, so I have to usually throw a blanket over the critter keeper or tub I feed them in so they can eat in peace.
As for the cats on top of the screen. First make sure that screen can bare their weight so they don't accidently fall through and the snake escapes. Either keep the door shut so the cat can't get in the room or keep a squirt bottle full of water handy. My cats are so used to the water bottle squit being their "no" signal that I can make the sound and they scatter.
Good luck.

-----
Chantel a.k.a. sillygirl
"I came, I saw, I adopted"
2.3.0 green anoles (Stumpy (M)), and the rest remain nameless.
2.2.0 green tree frogs (Romeo & Juliet, Bonnie & Clyde)
0.0.3 D. tinctorius (Cobalt froglets)
0.1 Anerythristic 'B' (Charcoal)corn snake (Popcorn)
0.1 Snow Corn (KandyKorn)
0.2 Amel Corns (KornSilk & KornMuffin)
0.0.1 Normal Corn (PepperKorn)
1.0 Creamsicle Corn (Kream)
0.0.1 Giant African Millipede
2.4 Domestic house cats (Bad Boy (m), Bart(m),Sasha(F),Little Bit(F),Spirit(F)
1.0 Rottweiler (Jake)
1.2 Rats (new additions with no names yet)
1.4 Mice (My step-daughter seems to think they are pets)
1.0 Husband (David, who loves all my critters too)