Disclaimer: I don't know / use the tool, but I'd like to highlight some general problems.
First, don't make this overly complex when troubleshooting. When the massren tool doesn't launch Vim as the editor, you should troubleshoot from the command-line, not by invoking the tool from within Vim.
Second, that tool (like any tool) needs to know when you are done editing the file list. Usually that is done by launching the editor in blocking mode, and just waiting until the editor is closed (this is easiest to implement). However, GVIM doesn't work like that, it returns immediately ("forks"), fooling the tool. With
--remote-silent-tab, this may even reuse an existing GVIM process, not spawn a new one at all.
Therefore, drop the
--remote-silent-tab and replace it with
-f GUI: Do not disconnect from the program that started Vim.
'f' stands for "foreground". If omitted, the GUI forks a new
process and exits the current one. "-f" should be used when
gvim is started by a program that will wait for the edit
session to finish (e.g., mail or readnews). If you want gvim
never to fork, include 'f' in 'guioptions' in your |gvimrc|.
If that still doesn't make the massren launch Vim, check how massren handles the editor launch; it might not support arguments (just a full process filespec), which you then need to workaround with a batch wrapper, and configure that for massren.