the only way to get a stable lens product will be making a professional injection mold and then having a plastic house manufacture them.
all the different resins will not hold up to exterior use with UV beating on it for years.
Lexan vacuum forming would work but you will need to invest in a professional vacuum table setup, and you will need to make the positives for the mold using plaster of paris.
or better would be investing in a cnc milled aluminum blank that you can pull 1000s of pull of lexan over. there are also different types of lexan that would need to be researched.
you will get into DOT certification as well since it will be a product mass produced for a car. it has to pass certain tests.
silicon does have a shelf life of less then a year. usually there is a date printed on the packaging letting you know the month of manufacture.
I've done many castings and pours over the years. to remove bubbles from a clear pour you will need a vaccum chamber as well.
i have a small chamber for my pours