Comp rules wrote:
115.2a Triggered abilities can trigger at any time, including while a spell is being cast, an ability is being activated, or a spell or ability is resolving. (See rule 603, “Handling Triggered Abilities.”) However, nothing actually happens at the time an ability triggers. Each time a player would receive priority, each ability that has triggered but hasn’t yet been put on the stack is put on the stack. See rule 115.5.
115.5 Each time a player would get priority, the game first performs all applicable state-based actions as a single event (see rule 704, “State-Based Actions”), then repeats this process until no state-based actions are performed. Then triggered abilities are put on the stack (see rule 603, “Handling Triggered Abilities”). These steps repeat in order until no further state-based actions are performed and no abilities trigger. Then the player who would have received priority does so.
603.4 A triggered ability may read “When/Whenever/At [trigger event], if [condition], [effect].” When the trigger event occurs, the ability checks whether the stated condition is true. The ability triggers only if it is; otherwise it does nothing. If the ability triggers, it checks the stated condition again as it resolves. If the condition isn’t true at that time, the ability is removed from the stack and does nothing. Note that this mirrors the check for legal targets. This rule is referred to as the “intervening ‘if’ clause” rule. (The word “if” has only its normal English meaning anywhere else in the text of a card; this rule only applies to an “if” that immediately follows a trigger condition.)
The answer to your corner case is... maybe. The infest would kill the Karmic Guide but not Stuffy Doll (yet). When it did, the trigger condition for lifeline would be true, so it would trigger, but not hit the stack yet. Then Stuffy Doll would die because it got smaller and that would not trigger lifeline. Then Lifeline's ability would go on the stack. Then spells and abilities can be played. If you can get another creature onto the field before Lifeline's ability resolves, you get back the guide. Otherwise the if clause is false on resolution and the ability does nothing.
Also, the outcome on the gatherer ruling is true, but the explanation is wrong. Lifeline doesn't save your creatures from wrath because it
never triggers, not because the condition is false on resolution. I should have read it more closely when I copied it.