Hi Simona, I have been thinking about your design all day. I know you showed me a sketch but the pictures make it easier to understand. It appears that you have created a recessed gusset. You have already made a recessed collar – the same technique applies to the gusset.
I think you would avoid the distortion of the lining if you notch your gusset and lining plus using stays.
It might be helpful to do a mock up – even in just paper to work out the sewing / construction sequence and writing it down so you put your bag together in the right order.
