This mead started bubbling like all hell after I took the above actions (pseudo-starter from other fermenting must, diluted, raised pH with very small amount of calcium carbonate). The OG is down from about 1.155 to about 1.040 (still bubbling every 10 seconds or so), meaning I've already beaten the yeast's alcohol tolerance (I've heard 71b, while listed at 14%, can get up to 18% or so).
Tasted a hydrometer reading last night and this stuff is delicious already, though too sweet at the moment. Thanks for your help. All 4 batches are getting down to their FGs, all is well.