I think I’m not understanding your question as the plugin doesn’t set any fonts, but uses whatever is set in the TinyMCE template. If there is no font set (whether by some plugin that provides fonts or by the text mode editor), then it will not set any fonts and the email client will decide what to set. I have yet to see Times New Roman being used in an email client.
Please contact me directly via email so we can schedule a screenshare session in order for me to better understand what is going on. vinny [at] usestrict [dot] net.