I'd probably only SvREFCNT_inc() if the "insert" code succeeded. That also means that you don't need to SvREFCNT_dec().
I think you'd be better off not having to remember to FREETMPS; LEAVE; if you return(), so I'd go for a simpler XS sub that lets the framework insert such stuff for you and call a regular C sub that can return() when it feels like it and the calling XS sub will do the proper clean-up.
demerphq is surely correct that this style isn't typical. But I beleive it is correct in its handling of ref counts and also understand the motivation for the style.