Google told me Solipsism is: "the view or theory that the self is all that can be known to exist."
That doesn't mean other people don't exist, just that they can't be known to exist by you.
Known: "recognised, familiar, or within the scope of knowledge."
I don't know where I am going with this.
I might be wrong and that short definition for Solipsism I gave is wrong, but I'm still interpreting it as meaning you cannot prove others exist, because you can always doubt it, and the only thing you cannot doubt is your existence because doubting is thought and you must exist to have thoughts. Obviously what I just wrote isn't indubitable (you just have to try hard enough), but I guess my point is that I think you don't know what Solipsism is... and I feel offended on the behalf of Solipsism for misinformation being spread. I guess.
ANYWAY, to answer your questions:
1. The self, obviously, it is in the definition of the theory. So depends who is reading. In my case it is me, since I am my self.
2. This is way to deep, so I won't bother trying to answer it and just say that I am probably not imagining myself. (imma proofreading right now and gonna say that I think this question was posed by you assuming you were the only one existing, which makes more sense to me, but why have that first question then? Also the only person existing doesn't necessarily use this site... but if they are reading this I guess they do, huh).
3. Only explanation is that Solipsism is wrong, hence you can prove others exist, since they post stupid shit, and you aren't stupid (no offence to anyone, I'm making a joke with a point here).
4. etc: "short for et cetera."
et cetera: "used at the end of a list to indicate that further, similar items are included."
Gonna assume you just want more thoughts about this whole theory, I gave them at the start of this, so I won't put them here again.
TL;DR
I wasted my time, if you read all this, you probably wasted yours too.
P.S.
To expand on my proofreading thoughts, if you/me/the self are the only person known to exist, the site isn't known to exist, so saying the only person who exists is a member of this site is a very questionable claim, since if the site doesn't exist (and you can't know it exists), then the statement can be false... you probs get what I mean.