Doing this pixel method to measure HP means that if others also damaged the target, then you will underestimate the amount of HP that the target has. In other words, say you did 1000 damage to get to the 2nd pixel, so that with 148 pixels, he should have around 1000*148 = 148000 HP. But unbeknownst to you, a groupmate did 200 damage while you did your 1000 damage, so in reality, it took 1200 damage to get to the 2nd pixel, so he would actually have 1200 * 148 = 177600 HP. Your measurement would be lower than the actual amount.
Anyway, it may have happened, it may have not. That's one limitation of the method, in that it doesn't account for if other people were also damage the target. When I test Arraetrikos's HP in part 4, I have everyone staying far away, so it's pretty clear that others aren't damaging him (unless they're ranging, but hopefully other groupmates would see and let me know that someone's doing that).
It looks like monster HP are still the same. Testing post-Update 5, the Jailer in part 1 of ToD has 88258 to 95163 on normal, which matches with the 90k amount from pre-Update 5, and 146668 to 160579 HP on elite, which matches with around 160k from pre-Update 5.
It was on an elite ToD run where I was the main tank. I've only finished analyzing the data for that one run, so I don't know about the other difficulties yet.
Horoth seems to double strike roughly 70% of the time or so. I've worked out that his DPS is roughly 130 DPS for just the physical damage attack part (which is around 100 DPS) and the damage over time effects (which total around 30 DPS if you have all three on you).