The established gravel and rings do have beneficial bacteria on them. But think about it - when you disconnect a canister filter, and you let it sit closed for a while, what happens? It starts to smell really bad. When you empty a fish tank and let your gravel sit with just enough water, it starts to smell really bad too. Why is this? Because oxygen is no longer passing through the porous media. Dead bacteria are a source of pollution, not purification.
When the power goes out, and is expected to be out for a while, the SOP for tending to your filter media is to take it out and put it in a shallow container of water with just enough water to keep the media saturated but not submerged, so that the filter media can "breathe" - it can stay viable for a while that way. But not *too* long. Not several days or anything.
Your few plant clippings are not providing the oxygen that motile creatures need. In fact, unless you are seeing the plants pearling wildly, I doubt they are providing much O2 at all.
So while your jar looks pretty, the same way flowers in a vase look pretty, it's not a suitable home for shrimp. Just leave plant clippings in there and it will look just as nice. If you want something to watch, try baby MTS (Malaysian trumpet snails) although honestly, it is cruel to close up anything. Especially if you have no equipment to test the dissolved O2 levels inside the container.