I've spent the better part of the past 3 seasons running the 3-2 almost exclusively. Over that time, my team was one of the better 3 point defensive teams in D2 Naismith. That being said, the 3-2 really only defends the wing and immediate front 3's well. Where I have been burned is the corner 3, and in particular against the triangle offense. I've seen it work well against all other types of offenses, but the 3-2 struggles with a good triangle team that has a good post threat combined with two good perimeter shooters. My experience is that you better have an extremely good SG and SF to run the 3-2 effectively. And by extremely good, I mean high ATH/SPD/DEF/WE. Your posts need to be high in ATH/SPD/SB too.

My opinion on your questions...
1. No, he's a better 2-3 player.
2.Yes, but you better have a really solid SG to help him out.
3. Personally, I'd play 2-3 + with your team...Miller's too good to have on the bench in D3 IMO.

I know coach_billyg has suggested that players have what basically amounts to a defensive quality score, which is computed from some combination of ath, speed, shot blocking, def. etc.  It may be situational (ie. shot blocking MIGHT matter more in the LP, or perhaps not), but if that's the case at all low speed will hurt you but can certainly be compensated by very much above-average ath/def for D3.  I have no doubt whatsoever that you're perimeter defense will be better in a 3-2 zone than a 2-3.  I'm still struggling to decide what that means for your optimal defensive configuration.  The fact that you're playing your team's BEST rebounder at SF means you give up a lot rebounding by switching to 3-2.  You still have a solid rebounding team, but it goes from being a team strength to a bit of a weakness given the ratings combined with a zone defense.  A lot of guys switch back and forth a lot between 3-2 and 2-3.  I've found that I tend to do better if I decide on the "optimal" or best configuration and play around primarily with the +/- settings.  On my Rochester team right now I have an SF who is nowhere near the rebounder that Miller is but who is still a well-above-average rebounder for a D3 SF.  He's also my best defensively player.  I have played exclusively in the 2-3 zone to take advantage of my guy's rebounding ability (my primary backup at SF is a "true" PF and also a solid rebounder).  This is in spite of the fact that my 2 post defenders are better than my guards are.  I guess the difference is that while my guards certainly aren't studs on the defensive end, they're probably about average for an established D3 program.  Yours aren't even at that level and need the help more than my guys do.  That said, I've had plenty of success with +3 and +4 2-3 zones in the past.  None of this is to say that if an opposing team was launching a silly number of 3s with at least one starting F/C I wouldn't consider switching to the 3-2, but it hasn't come up yet this season and in the past I tend to struggle switching over when I've been playing in my better set most of the time.