I've accepted that this show has no continuity or understanding of which ships are good counters for which and what constitutes armor and firepower. I think Rebels is ultimately a show that you have to sit back and enjoy for what it is despite how frustrating it can be. Ultimately I think the series will pay off and be good for the Nucanon.
As for Thrawn, I think this entire endeavor was to undermine Admiral Konstantine. I actually think he knew the other would fail and that this mission had nothing to do with stopping or trapping the rebels.