How do you apply fondant or gumpaste accents to a buttercream cake without denting in the buttercream?

Just gently stick them on there. You can put a little extra bc on the back of the fondant decoration, but usually I don't.

I put a little piping gel on the back of mine and press very gently.

If you pipe an open ring of b'cream on the decoration it will cause a 'suction' to help hold it on.

so adding the buttercream or piping gel to the back of the fondant accents will hold up to delivery and will not slide on the buttercream frosted cake?

..........so adding the buttercream or piping gel to the back of the fondant accents will hold up to delivery and will not slide on the buttercream frosted cake?...........

Yep. Did it many a time