Performing janaazah salaah alone behind the congregation

Q: In order for a person's janaazah salaah to be valid, does he have to join the saff, or can he read it individually behind the congregation? 

If he stands behind, and there is a large gap in between him and the saffs, will it be valid? 

Sometimes, the janaazah salaah takes place in the musjid which I know is not allowed, can I then join from the wudhu khana which will be quite a distance behind the rest of the congregation?

Zakaat on a gold plated necklace

Q: I would like to query regarding 18k gold plated necklace. 

I previously was not aware that zakaat was due on my 18k gold plated necklace. I'm not sure how long I had it in my possession. I am just going to estimate 13 years. I recently also gave the necklace away within this current year. 

I am not sure how much the necklace weight is: will estimate 18 grams, since I am not sure of the percentage/weight of the "gold" in it. 

Will it be okay if I calculate the zakaat on the current selling price of the necklace e.g. The current selling price is R1200 and then to make up for the 13 years =R 15 500 and then pay 2.5% of this amount, will this be allowed ?