Zakaat and lost jewellery

As-salaamu a’laikum

I have a gold chain in my possession, but it’s lost somewhere at home. It is an old gold chain of my mothers, but it is all in pieces (broken). So I know I have it, I just don’t know where it is. My other jewelry is cosmetic, sterling sliver and gold plated. Is sterling silver and gold plated jewelry zakaatable? Also how would you advise I calculate the zakaat on that broken gold chain that I misplaced?
I had also forgotten about a kruger coin that my grandparents gave. That coin is to be shared equally among myself, my sister and my brother. So I had forgotten about that coin too.

I hadn’t realized that I had to pay zakaat because I had forgotten about this jewelry, and the coin and I had no savings (Only a student). Now I must pay zakaat, but can I pay for all the years that I should have paid but didn’t?

Also is it fine to deposit zakaat money to organisations like Islamic Relief that has zakaat accouts?
Is that permissible?

Please let me know,

Jazakallahu Khair