I was wondering if there is any point at investing in spell damage if you plan on only using quen, with no igni or yrden. Is there any damage added to the reflected damage of quen?
(I know one way to improve it is a talent to increase numer of enemies it reflects on, I'm wondering about the straight +x sign damage).

Also, will Aard have added damage if you have +x sign damage.

Can quen cause critical effects when combined with the spell skill giving criticals?

Thank you !

I take it nobody knows. Devs ?

The devs, if they have any time at all, hang out at the official Witcher forums. Also a lot of traffic went there since they got it up, the guys over there might know more.

Sign damage only refers to offensive Signs like Aard and Igni(which are among the most commonly used) so no.