Lance is listed as a two handed weapon, and I don't see any way to use it with a shield. Since there are both mounted Paladins and Cavaliers using a lance and shield would seem to be a normal thing to do together on a mount.
Mounted lance combat got no significant attention at all even though both mounted Paladins and Cavaliers are in the Playtest Rulebook.
There's no damage advantage for using a lance while mounted, which is what it's made for.
If having extra damage with a lance while mounted seemed unbalanced before, maybe add or consider the following:
* Extra damage should only result from long straight line charge.
* The extra damage should only result when the lancer ends movement past the target's square.
* Setting a spear against a charging lancer should do similar amounts of damage as the lancer does from a mounted charge.
* Someone falling off a charging mount should receive more damage than falling off a stationary mount.
* Charging while mounted though a group of creatures should provoke attacks of opportunity from creatures other than the target.