Can one create a shortcut for inserting checkmarks?

Yes.  As Jon said use custom Snippets.  To implement, type your trigger followed by TAB key.

Sure, using your Snippets window, you can create all kinds of reusable code that can be dropped into your page via keyword shortcuts (similar to Emmet) or by clicking where you want the code and doubleclicking the Snippet Name in the Snippet Window...

1. Open the Snippets Window (Window > Snippets or Shift + F9)

2. Click the New Snippet icon (a page with + at the bottom of the window)
3. Give the Snippet a name
4. Give the Snippet a Trigger Key (you type this followed by Tab in Source View to drop your code in place)
5. Choose Insert Block for the Snippet Type
6. Paste the code you want to reuse into Insert Code

From there on out, you can click wherever you want the code to appear in Code View, type your Trigger Key and hit Tab, or doubleclick the Snippet Name, and the code will insert itself.

It used to be a bit easier with DW. We could assign true keyboard shortcuts to Snippets, but Adobe broke the functionality when they added Brackets and Emmet to the program for a few versions. Now it seems to need to use Emmet's abbreviation+Tab method to add the code. Not ideal, but still pretty quick for big code chunks that you reuse a lot.