My take on the whole size thing is that reach is a body proportion thing. And you can't compensate for body proportions. Once you've tapped out bike adjustments, ie. going to a shorter stem will compromise handling, the bike has to fit. If it still doesn't, it's too big and shouldn't be used.
Drop on the other hand is a mobility thing. Typically going a bit lower can be achieved by increasing mobility and/or easing other bike adjustments so dropping the saddle a bit.
Too much drop is also far easier to correct than too much reach by using a riser stem etc. I understand in this case that's not an option.
All in all, if neither size works with the given integrated components, I'd look for something else.