Shield doesn't make any difference for lancing.
The only difference is the types of weapons you usually use.
1) Lance + 2slot polearm.
2) Lance + Shield + 1slot polearm.
3) Lance + Shield + 1h.
In 3rd option you'd have to split your wpf between polearms and 1h.
there is no added shieldpenalty on top of the mounted penalty you recieve.
Because it's the same penalty: holding polearm with 1h instead of both hands, imo