I have 44/28 on my marin. It's as you plan, a triple with the middle ring deleted, and shorter chainring bolts. It shifts really well. I only did it though, because the middle ring was worn. I would rather have the middle ring too.
I think for this kind of setup you much rather have a friction shifter, so you can shift without dropping the chain, and trim, in which case, why not just run the triple? The whole super wide range double thing is slightly bizarre to me. It's just a triple with a hole in the range. Ostensibly it's for people who want to run 1x but with hill gears too, but you can ride a triple like that. And you end up trimming anyway. I think the niche which might make sense is if you really want to make it work with an indexed STI road lever but that might not even work so good.
Again, just why? Triples aren't cool? They are.
That
FC-M730 is a very nice crank too btw.
As for spindle length, I try to get it as narrow as possible. Experiment. Buy a few bbs if you have to. You don't want to be pressing the crank on and off but you can put it on loosely and eyeball it going on a bit further.
This is only because the middle ring was toast.