Can I submit my manuscript to a second journal without receiving withdrawal confirmation from the first journal?
This is actually a really interesting question as I don't think there's a black and white answer to this. Ethically, a "reasonable" attempt must be made to confirm withdrawal of the manuscript. People will draw the line at different points. 

I would argue that one attempt at contact is not sufficient to be considered reasonable. If possible, I would contact the editor by phone as well as email. Check to see if there's a withdrawal protocol through your online account or if there are any additional contact details of committee members. 

I would only feel comfortable submitting elsewhere if I had made multiple attempts to contact the journal over a period of 4-6 weeks. But that is just my opinion. I'm interested to see what others think...