Like the title says, is it not a bad idea to be using make tools for a directx project?

I'm thinking modifications to Visual Studio project files by multiple devs on multiple computers trying to push changes to GitHub might cause unwanted conflicts in said project files.

And how does using Make solve the problem of devs modifying files they shouldn't? The answer is that project file changes, when necessary, should be considered a part of change-set, just like any other file.

Combine that with the good (and necessary) practice of having developer branches merge into a staging area before being promoted into the dev mainline branch, and you should be well insulated from such folly.

So, the other devs should set up their own fork of the repo and I (as main dev) fetch changes and merge those deemed worthy?

You most definitely do not want all devs checking into one main branch without checks or oversight. Eventually good changes will end up in your main 'stable' branch, but you generally should have an intermediate branch of some kind where developers can integrate their work and test it thoroughly before potentially tainting the stable branch. You should read up on Git workflows (and Mercurial workflows, as it works similar to Git), there are a variety of ways to organize your projects version control.

If you don't have a proper way to filter and vet good revisions before promoting them towards mainline, then a version control system is little better than regular backups.

Since I don't work with other developers now and its just me, my projects have a stable branch, a dev branch, and then small working branches for myself. If it were more than just me, there would be branches for each developer, and they would have multiple branches of their own. But that represents only one of many work flows you could choose.