I'm using CLI mergetool vimdiff and rather than going line by line and typing :diffg RE for every change to select the REMOTE version, is there a way I can just have the REMOTE version of the entire file as the target merge?
Use :%diffget to get all chunks.
diffget takes - as most vim commands - a range. To quote vimhelp:
:diffg :diffget
:[range]diffg[et] [bufspec]
Modify the current buffer to undo difference with another
buffer. [...]
See below for [range].
[...]
When no [range] is given, the diff at the cursor position or just above it is
affected. When [range] is used, Vim tries to only put or get the specified
lines.
% when used as a range is "equal to 1,$ (the entire file)", see :help :%.