Borrowing for an event is very different from leaving a club and wanting to use it for the rest of the year. Calsouth assumes when you register you are choosing that club for the year and if you chose to leave early then you obtain a player pass or summer card. No manager should be giving cards to players leaving a club.
Are the insurance issues different? It seems like they would be the same to me. If you paid for the year, then you paid for the year. If you were injured while playing for another club, then you were injured while playing for another club. Why does the timing matter?