I think you're confusing who 本人 is referring to. Looking at the context, the speaker is probably talking about a third party rather than the listener. I think we should break the sentence down a little more. It looks like you understand the first sentence, so:
Yeah, well, I intend to just watch over (that person).
Although this sentence doesn't say who they are watching over, it implies that there is someone that they are watching over. So that person is who 本人(the person themself) is referring to. So,
||The person him/herself + subject marker
||Somehow / Some way or another
||Have no choice
||"So". Shows cause and result
So if you string the first three together, you get:
The person does something about it themself.
Adding the next「しかない」, meaning "having no choice" after a verb, you get:
Have no choice but to do something about it themself.
Adding こと nominalizes the phrase:
Something that the person has no choice but to do something about it themself.
And then finally adding なので just gives you a hanging "so..."
It's something that the person has to something about it themself, so...
Maybe that's not the most natural translation, but I hope you get the gist of it. This second sentence explains the reason why they are just watching over the person in question. You could probably flip the two sentences over and get:
(It's something) the person has to do something about it themselves, so I intend to just watch over them.
also I really just wanted to use the tables cause they looked cool