1. Are you asking if he could defeat anyone without using Kurama's power? Well he's stronger than the Gokage without Kurama. So yes, but anyone on his level and above would cause him to power-up.
2. Well Boruto through Sasuke's help took out one of Momoshiki's Rinnegan, the one that has the power to absorb Ninjutsu. He only really used the Vanishing Rasengan to stop Momoshiki from killing his father and the Gokage who were stuck via Shadow Possession circle.
3. That depends I guess. Momoshiki would realize that when it turns invisible, it's still prominently there or will attack him in that spot later on. Depends if Boruto can use clones to distract his opponent or not while he makes the Vanishing Rasengan and throws it his way. The cool thing is that Boruto curved it the first time he threw it for Sasuke, so it's more useful than we originally thought. IF Naruto had that ability he would be a bit OP. Momo'Kinshiki didn't even sense the attack, so given that understanding, if Naruto had created a Rasengan this
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and the opponent didn't know it was coming.
It would be a GG.
