C-x v v does not call hg commit as you'd do from the terminal. Instead it lets you enter a message first, and only then runs hg commit -m MESSAGE with the whole message.
Hence, if you'd like to have some message in that buffer, you'd need to implement it yourself. You'd need to define a new major mode vc-hg-log-edit-mode, deriving from log-edit-mode. VC ...
Not directly an answer to your question, but I would try out editorconfig:
This will allow you to add a config file which will set line endings, line length, indent style, and more and have it respected by every editor with a plugin.
It has noting to do with Emacs which works as a simple text editor in this case. If you want to abort merge, don't edit/save any file. Just abort text editor and hg/git will ask you do you want to abort merge. Say yes.
I assume your mercurial setup is same as https://www.mercurial-scm.org/wiki/MergingWithEmacs
I am not sure whether this is exactly what you ask for, but...
M-x monky-log, find appropriate changeset, click Enter
(monky is a magit clone, fairly nice, give it a try if you haven't tried it)
M-x xhg-log, move to appropriate changeset, and press =
(xhg is another sensible mercurial mode for emacs)
M-x ahg-diff (will prompt for revisions and display ...