Oh Pam, this breaks my heart. So sorry to hear this about sweet Frankie. One of my favorites here as you know.
It does sound like you are doing everything right. Reducing his weight will help quite a bit. The lasix will work great for him, but yes, it will make him pee. A lot. As the heart failure becomes more managed, his peeing will decrease and his breathing will improve. It is a manageable disease.
As for his food.... I would hate to see him go off the ZP since its such a great food. Which one does he eat? I would guess that the venison would be leaner and less rich than the lamb. They are also supposed to be coming out with a rabbit version. You might be able to do the green bean diet with him with NO sodium green beans. I know a lot of people have had success with it.
I'm not much help but want you to know that I am thinking of you guys and praying that Frankie will do fine.