Yes there will be some spanwise flow which will cotribute to the restoring moment, although due to the relatively small sideslip velocities this will probably be small compared to the overall lift.
There are so many different factors affecting lateral stability, and it is obviously the sum of all these factors that dictate how the aircraft behaves. The spanwise flow is just a small one. With reference to the oncoming forward flow also remember that in a bank, due to dihedral, the lift vector will be titled further from the vertical on the higher wing than the lower wing. This will give a greater component of lift from the lower wing producing a restoring moment.
Lateral stability is always a pig to try and understand, and i get confused everytime i try and think a problem through. I think im getting myself even more muddled as I speak so it might be best to ask an instructor/mentor or anyone who is more current on it.
If you have access to a good library try getting a hold of 'Aircraft Dynamic Stability and response'(1980) by Babister, or 'Performance and Stability of Aircraft' (1998) by Russell. I seem to remember both these books will give details all the stability derivatives (particulary Lv which is the rolling moment due to sideslip) which is what you are concerned with. They are both quite 'mathy' books however, and unless you're really interested they're probably not worth bothering to find and trying to understand!