Icing (as in sugar icing) moulds for cakemaking?

That might depend on what is in the envelope

They might be for making those shapes to stick onto mirror frames etc, as decoration.

A sort of putty was used and moulded, then allowed to dry and harden. Then they were glued onto the wooden frame and the whole thing painted. The result looked like carved wooden pieces, but were much quicker and much less labour intensive to make.

Lots of complex shapes were made this way – leaves etc. "Salvage Hunters: The Restorers" often feature a sculptor and a lady who restores mirror frames, and they both mould decoration pieces in this way to give the appearance of carved items.

Having said all that, it seems odd that only a couple of moulds are in this particular kit. It could instead have been used to mould sweets perhaps?

Not a clue!

Judging by the number of pieces, I suspect it may be components from more than one craft.

For a moulding set, I would expect to see more moulds. Although they could have been lost over time. What look like diagonal holes in the moulds, look a bit strange.

Like MJ1, the only things that look familiar, are what appear to be fillet smoothers.