If a DT1 user doesn't post for a couple of days around the start of the month then they miss out on being in the group that are randomly selected for DT1. If they aren't selected for DT1 then they would be DT2 under a few different circumstances so their trust feedbacks would still be visible. (Any feedback is visible if you know where to look)
I believe that you only need to post once in the last 30 days. However, you do have to be active within the last few days.
It’s 3 days of inactivity that will cost you your coveted spot
You’re right though, you only need to post once a month.