Is it OK to send another reminder after prospective research leader stops responding to my email?
You can wait for another 1-2 days and send a reminder again. In the reminder, ask if there is any change in the prospect of the planned collaboration. You can also offer yourself to help with the situation, if they see you fit for this (even if the chances are small that you can actually help).

Of course, keep being polite (but do not over-do it).

Good note from @user2912328 in the comments: verify the correctness of the e-mail address of the PhD guy.

Note from me: please also verify the SPAM folder of your email. A friend of mine found (by mistake / luck) no less than 3 messages from me hidden there - while I was worried that he might be upset with me or something.