How are these non-affiliated Setians viewed?
You didn't actually specify how they're being viewed by whom. In case you're asking the official TOS stance, there are some old Usenet postings by Michael Aquino where he clearly stated that the TOS were "the sole and exclusive initiatory agency for Set on Earth". That's as official as it can get. The essence is, as I understand it, if someone hasn't been initiated to Set by means of TOS membership, then he/she isn't a Setian.
However, this is only relevant inside the TOS. For Setians who have never been TOS members, this has no particular relevance. Maybe I should mention that I'm a Setian without ever even having applied to the TOS.. ^^
However, if you are approaching Set without the TOS, then it's entirely possible, but just be prepared for some extra rounds of slap on behalf of Set. Talk of a learning curve, or so. Whether you'd like some extra smacks depends on what you're expecting. It might be some extra sucks, or it might be worthwhile experiences. Who knows..