about the 2nd question, from my observation, the same monster types tend to have different health pools. the difference can actually be pretty significant.
dont remember what difficulty it was, think it was t6 and i was in a rift with lots of khazras. some had 17million HP while others had well over 60million.
also you should get a quezacotl for your jade harvester, from what i have heard it basicly double or tripples your jade harvester bonus dmg because it only damages for 1 minute and not the full 5 minutes of creeping death.