Never did it after barley but have wheat.  We would typically be 5-7 days earlier than you on plant date but have went as far as the 4th with milo and our first freeze is almost the same time frame too.  With drying capacity no need to worry about it ever drying down so that's good and if you are doing food grade you stand a better chance of hitting that i would think if you could harvest and dry it.  Our tw was always 54-56 unless an early freeze which i think happened once?????   What's plan B if you miss the food grade?  I'd definitely have plan B ready.

We put down 50-60 units of N shooting for 60-70 bu milo but hit 80-100 if it actually rained in August.  30 bu if it didn't rain but over the decade we did it we had a 65 bu avg.   We dropped 70,000 and i believe the variety was DeKalb 28e....big open head and dried down good but whatever you want on variety. 

Allelopathy issue, yes.  

Oh the row head, yes it feeds great and can pick up down milo if needed but i'd take the draper.  Seems like that short double crop plant always stood well.  

For us back then, double crop milo was the money maker to planting wheat and we never planted wheat w/out having double crop supplies lined up and ready to roll once the wheat was harvested.