I think the 0.9 ohm ccell is one of the highest resistance ones that'll work, but I'm not that familiar with Atlantis or Triton coils either, and will be one of the best choices available for tootle puffin. You can MTL with it in the M3M, at pretty low power, (20W-35W).
I don't know if the "weak battery" double-press to fire thing is a "feature" or a bug. It could be the former, if Eleaf wanted to discourage anyone from trying to use the mod on high power settings with a significantly depleted battery. Regardless, a firmware update could potentially change that. I've only gone up to 35W so far with mine, since I put a ccell in first, and I was able to take my batt, (VTC5), down to ~3.5V without this "feature" popping up. I'd take it lower, but I'd rather extend my battery's life, and I always carry a charged spare.