Here is my take on break in & what I used to explain to my clients:
No, if the saddle drastically doesn’t fit, no “break in” is going to help it. Break in only really means breaking in the leather. Any new saddle is going to be “stiff”, even if its made if the purest, highest quality calf skin. The panels and other parts of the saddle aren’t made of the same leather, they are made of a thicker leather. So, just like when you get a new pair of boots with a thicker leather, it needs to stretch and “break in”. That can mean that in 2 weeks it might sit a little different on the horse, but that doesn’t mean that it’s going to magically fit the horse if it doesn’t fit right now.
Small things like slippage, mild tightness around the scapula/shoulder area, etc could change a little. That’s why there is a difference when you sit in a saddle of the same exact model/panel/year thats one day old vs 6 months old. You might find the saddle isn’t as soft and buttery as the demo you sat in, and that will come with riding. I sent most of my demos to busy pros for 2wks+ to break in for me, so they were more comfortable for people to sit in and weren’t so stiff.
As for tree breaking in, it’s not going to break in. The leather will break in, the foam panel with compress in some areas, but they tree isn’t going to and shouldn’t break in.