Actually seniors can impeach leader after 7 days of leader inactivity, 8 days for Anbu. Not sure about below Anbu position. It cost 120 ingots.
Long ago we had similar situation, and I instantly created another group with alt and moved every member to it. We moved from lvl5 group to lvl1 again, and in few weeks we passed others groups and became Top1 again. It was a lot of work to do, but everyday you stay in dead group, you lose progress. (Goes away contributions, no GNWS, summons, group skills)