With my ride bindings and any board I have (other than my Burton) I can move my bindings 2mm forward or back if I need to. If they need to go more than an inch I have to change holes but I believe I get about 1 inch of adjustment nose to tail. If that doesn't make sense I'll try to get a picture, reading back over it almost confuses me lol.
Well maybe I'm special but my boots sit centered on the board with my bindings like that. If I had bigger boots I could adjust the heel hoops out to compensate. I suppose that if it was a real dilemma and still wouldn't work like that I'd be stuck with only 1" increments and I'd be screwed I've just never had a problem running my baseplates like that and didn't know they were on sideways I do have to mount them to my Burton like that as well. My pair of SP bindings have crazy baseplates that have a honeycomb look to them with about 20 (guessing, didn't count) holes in them so I can mount in all kinds of crazy positions.
I bought a used board with some of those honey comb baseplates and the guy had put 5 screws into the burton board all in a weird pattern. It was quite the mess when I got it. He had also melted about 5 bars of wax onto it and never scrapped it.
Well if the set back stance is more comfortable and you don't plan on riding switch that would be the way to go. If you want the bindings centered on the board you may have to deal with a less than ideal stance width. I'd just try them both ways even if it's in your living room to see what feels more comfortable and go from there. If you don't like it you can always change it in a few minutes at the mountain.
Clearly I cannot change the insert pattern on my board, so should I chose to set the front binding back though it will give me more setback?
If you don't really ride switch all that often, that would definitely be the route i would go. If you do ride switch often, you would probably prefer the stance that is closer to centered. Either way you can achieve 20" wide, if that's what's most comfortable for you.