How to show Import Workflow step only for one period

One option is to let people see the step, set  "Is Optional Data Load" as True, then have a Confirmation Rule at the parent level that will check, on M12, if the activity has been done (or conversely, alarm if it's been done at the wrong time). This is probably the lowest-code approach.

Another option (arguably the cleanest) is to set the Form to be a Dashboard, which will manage the load "behind the scenes" with rules. That allows you to leverage the time filter on form templates, but you will have to reimplement some of the Import interface (report validation errors etc).

Another approach (possibly even in combination with the one above) is to load data in a completely different Scenario/Workflow combination (let's call it "Adjusted"), which could be set as Hybrid to sync initial data from the main one (which we'll call "Main"). You could use rules on Main to pull data back in from Adjusted on Consolidation. The Workflow Tracking Frequency of Adjusted would be set to Yearly, and probably should have some sort of Event Handler to allow loads only when it's M12.

I have a feeling that there are also simpler solutions, leveraging channels or Marketplace Solutions, but they're not coming to my mind at the moment.