I have Max and Mary. I have found it a useful tool in the past, but to be honest, I do not think I have used it since I got BibleWorks. I'm sure I will use it in the future, but its usefulness is somewhat limited now. So to answer your questions,
1. Yes to Max and Mary, but I would not call it indispensable for one who has BW or similar software readily available. It does provides analysis that BW doesn't.
2. I do not have Zerwick's grammar, but I probably will at some point. Max and Mary is keyed to Zerwick's grammar, so if you want to see how they define the syntactical terms they are using, you will have to look it up in his grammar.
3. I think I kind of answered your question above. But if you just want a Greek grammar, get Zerwick's grammar. If you want a verse-by-verse analysis of the NT, get Max and Mary. And, as noted above, the grammar is a valuable supplement to Max and Mary.
I hope this is of some help.
καὶ ὑπὲρ πάντων ἀπέθανεν ἵνα οἱ ζῶντες μηκέτι ἑαυτοῖς ζῶσιν, ἀλλὰ τῷ ὑπὲρ αὐτῶν ἀποθανόντι καὶ ἐγερθέντι.