I vote for going ahead with the surpise party. Even IF the OW gives it away, your H will still know that it was her that did that and that YOU still planned everything and got it put together. Just like all the other areas of your life, go on as if she isn't there, cuz she really isn't any way. Focus on your H and if he would appreciate a party then bring it on!!
As for the gift, I vote for non glitzy. From my cyber knowledge of you, it doesn't seem to be your style. And your H will know it is not from YOU, but an alterior motive. Even if he can't quite put his finger on what it is about it.
I say find something that is specific to something that you two have shared. Like something he saw while the two of you were away, that ONLY you would have been able to share with your H. Even if it was from years back. Something like, "Remember when we went to XXXX and we saw YYYY and I wanted to get that special thing for you on this special day.....
