Arguably Doom didn't hack former Ultron, he just "fixed" broken Ultron. Hank had his hands on Ultron's former bodies and he still had to use physical infiltration and an old programing language to stop him by using a virus. Again, we can agree Criti-Knoll is no Doom, but since Ultron improves itself quite totally each time, using Doom's experienced to hack Ultron isn't necesarily something feasible without prep.
Regarding summoning the Mindless ones, I agree that Doom did it under his own power, however, I'm torn in whether he used prep or not to summon them, considering that he came from the past after spending an undisclosed amount of time learning and -possibly- prepping the spell before going back to his castle. Admittedly it could be just a spell he can throw at random given that Doom's magical ability has been displayed to much more depth recently.
I agree with the rest of your statements.