I'm narrating a story about events which happened yesterday. I'm looking back on completed events, so I use perfective.

Suppose I write: 

{wa'Hu', qaStaHvIS wej rep, jIQuchpu'}

What does it mean ? "yesterday, for the duration of three hours, I've been happy" (i.e. my happiness lasted three hours), or does it mean that during those three hours, I was happy only once (for a period of time shorter than three hours, e.g. a minute, an hour etc) ? Or can it mean too, that I was happy more than once, i.e. my happiness going on-and-off ?

Don't say this. Say: wa'Hu', qaStaHvIS wej rep, jIQuch.

It's not about whether you're looking back on an event; it's whether you're describing an event as completed. Here, you're not describing your happiness as completed;  you're describing it as occurring during a particular time. That your happiness may have subsequently ended is irrelevant. You're describing the moment it was true, not the condition after it came to an end.

I'm not going to go into another conversation about what perfective might mean on verbs expressing qualities.

-- 
SuStel
http://trimboli.name