The Lost Coin is a parable. There is no evidence that Jesus was referring to Mary specifically any more than lost sheep referred to her.
Luke 15:8-10
Re: Mary, Joseph, and Jesus being poor
While Mary was indeed humble we do not know if she was poor. She was betrothed to a respectable man with a respected occupation that required training and likely apprenticeship. ( Was a dowry needed in those times? ) She did travel to see her cousin. Presumably when the went to the inn to find shelter, they were likely prepared to **pay** for their night's stay in the inn. They were able to travel to the temple with Jesus when he was an infant and as a boy and I assume that they had the provisions to do this safely. They had the means to flee to Egypt and to stay there a long time. So? Should we conclude that they were poor by that day's standards? Personally, my conclusion is that they had enough wealth to provide for a reasonably secure working ( middle) class life.
Re: Working Class
The middle classes work but I will not quibble with you over working class or middle class. If you wish to call the family of Jesus a working class family that is completely fine with me.
I suggest you take a look at what remains of the village of Capernaum. And why are you so adamant that he was NOT thinking about an event in his life? Do you think he just plucked the image out of thin air?