The slugs were probably mating and laying eggs? That was their last chance of mating i guess.

As for the frog, that sounded like how a frog sheds his old skin and is practically normal.

Hibiscus isn't toxic but their wood decays faster than most wood. Have you taken off the bark before putting it in?

Does your pond water smell funny? Other than that I think you may have a perfectly healthy pond that is experiencing some current climate changes (i'm thinking Au is hot this time of the year?)