Solipsism is incoherent because it's not radical skepticism. All of the critique of the external world also apply to belief in the primacy on internal experience. Any good solipsist should just accept the "evil demon" of descartes, embrace radical doubt, and say "I don't even know if I truly exist or not".
"I don't know if I'm a P zombie, and I don't know if I'm a replicant or not, Deckard!"
well doesn't the argument suggest the only thing you can be certain of is I, or at least some 'experiencing agent' exist, otherwise there would be no subject to do the experiencing
Yeah the first-person subjectivity has to arise before second and third persons can arise. But with some further investigation, one can find that the things they take to be their subject are in fact object to them, too.
"I don't know if I'm a P zombie, and I don't know if I'm a replicant or not, Deckard!"