I know it may seem dishonest at first, but your parents do not need to know everything that happens in your life. Some things are personal. Your parents seem like they mean well, the only discrepancy is that you do not want to always follow their religion. I think your doing good by laying low and playing it cool. Try to stay out of their way and use the excuse of being loaded with homework. If they don't like that then say that you can't wait till you get your degree so that you can help other witnesses, or whatever.
I was stupid and told them exactly what I thought of them and their religion. In the end, I lost all of my family and I'm paying for college myself as I also raise a family. If you can bite your tongue around them then I think you could do it. But in the long run, you will pay for college in one way or another, because they can lord over you with the fact that they paid for your schooling.
Can you go to a friends house, that is familiar with your situation, and have a small party there? But if you miss this party, you will always have time for many more parties once you leave the witnesses for good. So you may want to lay low for now. Love