The orbs damage goes up with level but stops improving in level 7 according to the description of the spell. What I find strange is that the chances of stun/paralyze is higher for my bard compared to my F/M/T and F/I. Their wizard levels already past level 9 (the bard is higher though). Has chance to stun/paralyze got to do with levels?

I don't think so. What makes you think that the chance to paralyze is higher for the bard? Observation? If so, I'd say it's random chance.

What should affect paralyzation is the enemies you are attacking. For example, you would see a difference if the bard was going after lizardmen and the other two were trying to stun yuan-ti.

One thing I have noticed is that incorporeal undead don't seem to be affected by the stun/paralysis effect of Chromatic Orb.

Yah. Observation on mostly on lizardmen and yuanti.

The spell description is silent on chances against levels.

I think that's how it is. As I understand it, the caster's level does not affect saves against a spell in 2nd Ed, in contrast to 3rd Ed.

If you've got some priest spells to burn, Recitation is always a solid choice.