There's clearance all the way down the middle of the saddle. I actually stick my crop in there while I ride because how else am I going to carry one? Hahaha So I know for sure that there's plenty of clearance in there.
I think in that picture bsms posted, the pad was pulled up (I hate it when the pad is right down on the withers) so it's making her withers look bigger than they are, especially with the way her mane is right there. But we'll see when I take more pictures tomorrow!
I never considered that about the width of the saddle! I'm pretty sure it's as wide as it can go in any case, I mean on the DownUnder website (where I got it from) they just had two options of "medium" and "wide". I am going to be really sad if this saddle doesn't actually fit her either... Saddles basically hate this poor horse.
Also, I am pretty sure that her shoulder does push the saddle back if it slides too far forward (like it does when we go downhill significantly) at least. I do try to set the saddle on behind her shoulder when I tack her up but her shoulders go really far back and the hills like to scoot her saddle farther and farther over her shoulders. After the hills I usually get off and readjust her saddle to it's proper place though. But if I don't slide it back, the saddle eventually works it's way back to the correct spot...
Hopefully I'm answering all the questions you have satisfactorily... :)