Is it possible to mark all articles in a group as “unread” in Feedly

Yes!

In Preferences | General, find "Filter Empty Collections" and set it to "On".

The hackish way to do it is to use the built in developer tools of a browser like Firefox with the Web Developer extension or Chrome with the built in Developer Tools.

In Chrome for example:

Open up Feedly

Go to View -> Developer -> Developer Tools This will bring up the Developer Tools bar at the bottom of your browser window.

Then hover over any of the unread count numbers you want to remove.

Right click on the number element and select 'Inspect Element'.

In the Developer Tools bar on the right hand side, you'll see the CSS style rules for the page appear. You should see one that says 'div.simpleUnreadCount'. Under this rule after the curly bracket that follows that div, type in: display: none;

You should see all the unread counts disappear from the page. This will only last as long as you keep that page open. You'll need to repeat these steps (quite quick once you know what to do) each time you close the page and then reopen.

OK, that's the hackish way to do it that at least shows you how it works, now I'll show you the easier way to do it:

Use a browser extension/tool such as Stylebot, Stylish or Stylist. These allow you to edit the CSS styles of a page and have it persist each time you visit. You'll want to tell them to do the same thing as above, but this time they'll save the changes and you can revisit the page and see no unread counts.