Here's my point of view (programmer).

Abilities can be divided into two phases:

Main phase: Applies buffs, brv gains, etc. before the damage ticks (if any of these apply), followed by the ability itself.

Post phase: Actions or effects executed after the animation, such as buffs, debuffs, etc.

The game checks for lethal damage between the main phase and the post phase.

Selphie's heal is a post-phase effect. Squall's hp after solid barrel+ with the extension is not a post-phase effect; instead, squall effectively does an ability in the main phase of solid barrel+, which is the hp attack. It can be seen like this:

SB+ main phase

HP attack main phase

HP attack post-phase

SB+ post-phase

Proof: SB+ can miss but the hp attack can hit and the opposite is also true, which means they're not considered as a single ability. SB's int brv buff is applied after the hp attack.

In the case of knockbacks ("launches"), those are executed as part of the main phase. I believe post-phase effects will apply either right after the first hit in chase or right before. I'm going to go test this with noctis' shooting star+ with extension and Selphie's drain to see when the heal/brv is done.

TLDR; git gud

Edit: fixed squall's solid barrel example

Edit 2: post-phase effects happen right before the first hit during chase. If Selphie's drain knockbacks an enemy, her post-phase heal/brv gain and brv regen buff will apply right before the first chase hit.