If I use all butter instead of butter/shortening how will it affect the texture of the cookie? Will it be flatter? crisper? Should I change o...

If you go full butter, your cookies will flatten/spread (potentially quite a bit). If you really, really want to do this, you will need to compensate in other parts of the recipe, such as adding a bit more flour or even switching to cake flour. I haven't read this recipe, but if it doesn't already have you putting the cookies in the fridge for an hour before baking (not the mass of dough, but the ready-to-go-in-the-oven cookies on the cookie pan) that can also help reduce spread.