Reviving a Rejected Proposal -- Question on Ethical Considerations
In the background that you have presented, I see only one half of the question being asked: "Will you join on this proposal?"

The clear next step is to query again with the other half, in order to make disinterest explicit, writing something along the lines of:


  Since you have not responded to my emails asking whether you would like to join this proposal, my understanding is that you are not interested.  I am intending to go ahead with this proposal, so unless I hear otherwise from you, I will assume you have no objection.


This places the question squarely with the other person.  If they respond one way or another, it resolves the question.  If they do not respond, you are free to assume that they are accepting that you submit without them.

Do, however, make certain to do due diligence to determine that the other person hasn't changed institution or email, such that your communications are not being received.  You may also want to try a phone call before giving up on contact.