Yes, I would start all over and forget about what she has been doing. pretend she is a new dog that you don't know the history of.
Food guarding: call her name and when she looks at you, give her something that she likes way better. Then walk away, no further rewards or praise so she learns it is part of life (again).
And never her board her with your BIL anymore.
But also, she may have hit the bratty stage in the meantime and good luck with that; I am in the middle of all that stuff.