Hi,
I can answer your second question based on own experience. I had a similar situation a couple of years back. And I switched to private insurance (if you do not have kids, the contribution for public and private insurance is almost equal for both husband and wife). As time passed, my company went bankrupt and I was jobless. That time, I could shift back to public insurance (AOK), since I was not insured privately for more than 5 years before unemployment.
So, the answer to your question is, "god forbid" if you become Arbeitslos in coming five years, you will be automatically switched to AOK by Arbeitsamt evenif you chose private insurance now. Only catch will be the Kundigungsfrist for the private insurance, otherwise you will be paying for both the insurances...

Hope this helps,
- Dushkal