A292: The clause "so long as the shield can be interposed between the wielder and the spell effect" is going to require some case-by-case adjudication. Losing your AC bonus suggests you don't have total freedom to position the shield that turn.

Concerning Spell Combat specifically, Bucklers don't actually make your hand free - rather, there is a specific list of actions/weapons you can use in conjunction with a buckler as though it were free, but Spell Combat isn't one of them. You would need Unhindering Shield for your hand to truly be free while using a buckler (and thus be able to use Spell Combat), and with that ability, you should be able to utilize the Ward Shield as well.