You are going to have a great looking quilt!
I don't think starching will help--that knit fabric will still stretch with it. Since you've already used interfacing, maybe a machine basting straight stitch along the sides of the T-shirt blocks will help reduce the stretch. Did you check the Tshirt blocks before you sewed them to the pieced blocks? Maybe they "grew" by stretching a little bit as well and could be recut to make sure they fit with the corner triangles. From here it looks like it's just those 2 squares which are put on point. Best of luck and show how it goes!