The best M↓ editor I've used is Haroo Pad[1]. I like it because you don't need to visit a webpage to use it which makes it ideal for offline idea dumps while you're on a plane for example.
I actually started off with Mou, but I've found Haroo to work a bit better for my needs. It's got better shortcuts (although I don't need them) the interface is a bit nicer and it's got more customization. Plus since Haroo is built using node-webkit, it runs cross platform.
Much the same, here. For some reason the font rendering on my Ubuntu machine seems sub-par (only in the editor, the preview pane looks good). The Windows and Mac versions don't have that problem.
I really don't there's a need for a special application for this; I use Emacs's markdown-mode, python-markdown, and xhtml2pdf along with a docview pane to edit and preview Markdown, and I can't imagine a smoother experience with a dedicated editor — plus I get to use the editor to which I'm generally accustomed.
You're right, you definitely don't need a dedicated application, but I would rather just open up one executable instead of cobbling together multiple utilities. It's all about your workflow though, for me, I feel more comfortable using a M↓ where scrolling is synced and it can watch what I'm doing in real-time.
I'm gonna look into your setup though. I'm definitely all about being on the terminal, does your setup work well with VIM as well?
Not really, since it relies on Emacs' docview mode, which can display the pages of a pdf file; I'm not aware of a way of doing that with vim. If I were doing it in the terminal, I'd probably use w3m in Emacs to roughly render the HTML. I don't know if that's possible in vim either, but it seems more likely.
I, too, have been playing around with Haroo Pad. During a typical day, I'm using Ubuntu, Windows, and OSX, so I really like the cross-platform availability. While I'm a paying user of Mou, and really like it, it is unfortunately OSX-only.
Congrats to the creator of StackEdit, it seems very well done. It also seems to work really well while offline, which is nice. The biggest blocker for me is that when I need to create or edit a markdown file, it's usually a local file in a git repo. In my case, it is simply more convenient to use a native app.
[1] - http://pad.haroopress.com/