Way to confuse me even more. Let me simplify my questions into a statement: CAP 22 and Serperior are opponents to each other, and CAP 22 uses Parting Shot. If Serperior can attack after a teammate of CAP 22 is brought in through Parting Shot, then it is necessary that CAP 22 has an advantageous match-up against Serperior, for the above scenario would mean Serperior can attack a Pokemon at +3 the same turn it is hit with Parting Shot. To simplify this even more, our CAP should be able to keep Serperior from switching in, or even from abusing this situation if such is possible.
Ok now I'm at a computer, so I can explain more thoroughly.
If you actually read what I said:
I think the free boost would be more of an annoyance than anything. If you switch in Serperior as CAP22 uses Parting Shot, you're sitting at +1, yes, but then your opponent has the switch advantage. Serperior's damage output is still a little underwhelming at +1 anyways.
This was the situation I was talking about. Serperior successfully gets the +1, but then you can switch out of it into some other check to Serperior.
However, I'll talk about all the situations. Note that I am assuming that CAP22 is faster than Serperior.
Situation 1: Serperior hard switches into CAP22
This is the situation I referred to in my quotation above. When Serperior switches in it gets hit by either a) a STAB Fairy- or Fighting-type move, b) Parting Shot, or c) some other move. If we go off what we know (situations a) and b)), a) Serperior either takes a STAB move, which it won't appreciate (and if we're faster it has to survive two STAB hits) OR b) it gets hit by Parting Shot, reaches +1, but then has to face one of CAP22's teammates, which could very well OHKO Serperior.
Situation 2: CAP22 hard switches into Serperior
This situation has no impact on our situations. Even if we're faster than Serperior and we can tank a Leaf Storm, no good player is going to actively use Parting Shot on Serperior.
Situation 3: Serperior comes in after one of its teammates faints
Again, no good player is going to actively use Parting Shot against a Serperior, so either CAP22 hard switches out or hits Serperior with a STAB boosted move.
So the only situation (assuming that CAP22 is faster) where Parting Shot's effects become a problem vs. Serperior is Situation 1, which can be easily circumvented with another teammate switching in. In Situations 2 and 3, if CAP22 actively uses Parting Shot against Serperior, the player is just using CAP22 stupidly.
EDIT:
HeaLnDeaL said exactly what I wanted to say with less words.