Yes, I thought toki utala was not a great fit, it is kind of a misuse of the term. "toki nasin" is a much better fit, I much prefer that term
Right the first three statements are merely describing what I mean by sewi, in this argument. It is less, what the proponent may or may not believe, but more something like ; for the purposes of this argument... So it is not circular, it is deductive. The steps of the argument, ie propositions 2 and on, come naturally from what we mean here by sewi.
So proposition number one is the key thing in the argument; it this even "possible". If it is, then it logically follows that sewi exists.
Proposition number three just follows logically from the conception of sewi. Hence if sewi exists in one possible world, then sewi exists in all of them. This is a bit tricky to follow, but it merely follows from what we mean by sewi.
A good argument is normally thought of as one that is valid, and has propositions that are more likely to be true than false. Since the only "input" proposition of the argument is the first one; and that merely states sewi is possible. I personally think the argument is a good one. Although, if you really look consider the argument you see it kind of plays with word uses a bit and the first premise is not as light as it may at first seem, so maybe the argument is more a good exercise than necessarily persuasive.
[added abit later, before any replies]
The argument itself, I particularly like because it explains what we mean by sewi. Also given you understand what we mean, it just logically follows that such an entity exists. Unless what we mean here has some kind of internal contradiction, which it seems not to have, then understanding this meaning, we should that sewi exists.
A circular argument is different from a deductive argument, although circular arguments be logically valid. A circular argument is one in which extracts no new information, that is to say that, the only grounds given for believing a premise is that you already believe the conclusion. A good deductive argument on the other hand, extracts information already contained in the premises, using the rules of logical inference. Which is to say that we might already believe the premises and not yet realise that it follows we should also believe something else; the conclusion.
Follower of the official dialect of toki pona as presented in the official book; Toki Pona, The Language of Good by Sonja Lang.