Planning For the Future, Ha Ha

Say an author has reached that happy place where multiple editors are interested in purchasing her debut novel. Would it be viewed as peculiar if the author wanted to interview each editor to find "the best fit" -- dollar signs aside? 

Not at all.  This doesn't always happen, but it happens often enough.  And the editors involved are usually really pleased when it does--it says you value our part in this process as well as the check we'll cut you.

Also, if, for instance, the debut novel were a Middle Grade and the author has hopes to one day move into the YA market, is it best to find an editor who handles both? Or is it enough to go with an editor whose publishing house handles both?

Publishing being what it is today, it's more important to give this one book the best publication it can have than what will happen after.  Success has a way of sorting itself out--and the market (and your publisher) may be different when you start in YA than it is now; things are so volatile in publishing.  Do the best you can for this book.  When (if) you write a YA novel, do what makes most sense for THAT book THEN.