I don't think I would allow a lance to be treated as wielded in two hands in all respects (except for the number of hands it takes to use) when mounted.[/quote[
I don't advocate that either.
If you're wielding it in one hand, it's treated as being wielded in one hand, not two hands.
I'm not referencing any rules that are dependent on a weapon being wielded in two hands, though. I'm referencing rules that are dependent on the lance being a
two-handed weapon.
Just as a longsword, according to the PHB, is a one-handed weapon whether you use one hand or two hands, the lance is a two-handed weapon whether you use one hand or two hands. It's just that no other two-handed weapon is
capable of being wielded in one hand.
The proposed formulation also implies that lances also get 1.5 times Strength bonus whether wielded in two hands or one hand. Again, this is somewhat counter-intuitive for me.
As long as it's your primary hand, yes.
If it's your off-hand, we have a contradiction. As a two-handed weapon, you apply 1.5x. But as a weapon wielded in your off-hand, you only apply .5x. The DM thus needs to determine which of these takes precedence.
As a DM, I'd rule that a lance wielded in the primary hand adds 1.5x Str bonus to damage, but a lance wielded in the off-hand only adds .5x.
-Hyp.