So you agreed that he handicapped himself in an attempt to do both?To me, the only thing that really hold him back was that he was fighting Minato AND controlling Kyuubi at the same time,
And the fact that Obito's priority was the destruction of the village, he did not want to let Kyuubi on the loose thus did not stop controlling it to have a better fight with Minato.
With that i mean that Obito couldnt use Genjutsu against Minato, or at least technically shouldnt have been able to, because ;
Minato himself mentionned that if Tobi was Madara, there is no way he could control Kyuubi for long. Meaning that Obito, even using everything that he had regarding to genjutsu power, wouldnt be able to control Kyuubi for long. Attempting to cast genjutsu on Minato while wanting to keep control of Kyuubi would have been impossible.
So to me, Obito's drawback was that he couldnt use genjutsu on Minato as killing wasnt his priority, keeping the Kyuubi under his control was.
So it's not plot no jutsu to me, it's that Obito had to make a choice... fight a better a fight against Minato and loose Kyuubi, or handicap himself and attempt to do both.