My shoe size is 7, so I'm not really qualified to say what the ride will be like with a 5.5 boot. For me at least, the 145 Infinity is perfect for a size 7 boot.
I guess since your foot is rather small, you can lean towards a smaller 142 board. It's probably the ideal size for your weight anyways. If you have no issues riding your board now, the Infinity might actually be easier, because it tends to be on the narrower side IIRC. So, I wouldn't worry too much about waist width if your current board isn't bugging you. Really, 0.2-0.4cm extra width isn't that much of a difference.
I suppose torsional flex might be a bigger concern, as other people said here. The Infinity is soft/medium flex between the bindings, so you'll probably be fine.
The Infinity is a great board, so you'll do fine, but if you want to look into others:
Endeavor Boyfriend or Diamond
Arbor Cadence (Twin, but works well all mountain)
Smokin Vixen (but somebody on this forum said it was squirrely)
But really, the Infinity might be the best choice out of all of those with all the concerns you mentioned.