We have purchased tickets for 4 for entrance tomorrow at 10am. Two of our guests do not have proof of vacination. Are they permitted to enter with masks? If not, are the tickets reuseable for another date?

It's not required

Np

It is an open space. I don't think they check the card. At least not for the garden and observatory

They didn't ask for it, masks only inside the museums.

No proof of vaccination is needed

Yes, they should be fine. :) I just went without a mask and without proof of vaccination and no one said a thing. You do need to wear masks inside all buildings, but not outside. :)