How to deal with email unresponsiveness? I sent a research mentor an email (admittedly quite lengthy and dense) which he hasn't responded to in several days, nor my request within the email to discuss via phone / video call. 

What should I do in this situation? Should I just resend the email?
Adapt the communications structure to the situation at hand.  

A request for a video/call conference should be sent alone, especially if your researcher seems to disdain long e-mails. In the 1st such live communication, clarify with him whether it will be pointless to send him long e-mails, and whether instead you will communicate live, mainly through video/call conferences.