I do not believe that anyone is pulling Trump's strings -- he is far too chaotic and unmoored from any fixed principles to make an actual plan workable -- but in the short term I think he is certainly useful to others in advancing their own agendae, and perfectly willing to be used (or, maybe more accurately, make common cause) if it satisfies his own narcissistic needs.
I agree with you in part--he is far too chaotic and unmoored to make any plan work, but I think this also makes him impossible to come up with a plan. And that is where the string pullers come in. Granted, he can be a problem (I still think he could have ended Obamacare if he kept his big mouth shut, but he gets them seats at the table--seats they'd never get on their own (or with any mainstream candidate. Face it, he was shooting his mouth off for years, stating idiotic political philosophies, but he wasn't taken remotely seriously until he teamed up with them. And they got him in with the right groups to play to his ego and promote him. Face it, without them, he's the flipside of Marjorie Taylor Green (and without him, she'd look like an even bigger asshole--and that ain't easy.