i think kacheek life potion is the only exception because of the special/unique healing requirement, from my experience too (33% of Max HP when you have 33% or less of your Max HP,
Heal Full when you're over 33%)
yes, drain life messes up the 66.7% heal of KLP (makes it 33.3%)
good to know in 2-p fights
therefore, I'd say drain happens before everything else, except that it also 'overheals' (kinda like, drain 100, if extra, add a 'shield' type of thing)
I think the way it works is that direct damage actually does negative healing. So whatever the heal function is in the Battledome, the CBT heals 100 for the user and -100 for the opponent. You'll notice this when someone uses a GHS and Drain Life at the same time. Say both people use a GHS, and drain life does 30 damage. The user would heal 80 HP, and the opponent would heal 20. It would actually look like that on the results write-up.
Well I just did testing on KLP. Take a look for yourself:
KLP_1 (MAX HP, Drain attacks first, click 'GO' second)
KLP_2 (MAX HP, Drain attacks second, click 'GO' first)
KLP2_1 (33.3% Heal, Drain attacks first, click 'GO' second)
KLP2_2 (33.3% Heal, Drain attacks second, click 'GO' first)
Conclusion: When above 33.3% health, the user overhealed the drain. (will always be max HP). When under 33.3% health, the drain damage "cuts in" to the heal. In KLP2_2, the opponent SHOULD have healed 86 HP, but instead, the drain damage is cut into it's effect, and it only healed 78.77.
*Although it says that Carnbuckle cut into KLP AND healed itself, it is unlikely any healing is actually taking place.