You can blockmix a ring cp into any other block, but that does not mean that the other block turns into a banked block. Ring cp and flat block are still two different blocks, even though they're build in the same grid position.
And no, mixing in a flat cp at that part is not vaild either, since the flat block at the position stays flat, while only the cp would be allowed.
I'm a bit surprised actually that you didn't ask for this in the post because a lot of people asked for combinations that were a lot less complicated.
If the deadline wasn't reached yet, you sure could've fixed it, but sadly it is too late for that now.
I can understand that you're frustrated now.
I hope you will still participate in future MTCs