Lyall is an all-suite, boutique hotel that is "luxury" for that category. I've just stayed there for a week while some fire damage was repaired in my home. Rooms are small, although well appointed, but if your idea of luxury is Aman you are unlikely to rate the Lyall as luxury. That said, it's really comfortable, conveniently located in South Yarra and therefore could be a good option if you need to base yourself in Melbourne for an extended period.

Park Hyatt is on the edge of the CD so it's an easy stroll to the "better" end of town, where lots of great restaurants and bars can be found. Como is too far to walk unless you are a pretty serious walker.

I agree that the Prince (which I like) is probably not the right hotel for one's in-laws! It's in buzzy St Kilda which is fun but gets too noisy at night, and there's not enough bench space in the bathrooms

It sounds as if you like boutique hotels, and most of the hotel development in Melbourne recently has been in this segment. Have you thought about the Olsen, mentioned by number_6 (over the road from the Como)? I don't have any personal experience of it but it could be worth investigating.