So we need two feats - TWF and ISB - but taking these, which are a given for sword-and-boarders, merely give you -2 to hit when using the Defender ability. A small price to pay for +11 AC?
As long as you're taking the full attack action, you could make a shield bash. Even if it's attacking into a square against the possibility of an invisible opponent creeping up on you.
If you're only taking the attack action, then no, you're not actually making a shield bash, so by using the shield as a weapon the shield bonus is forfeited despite ISB.