SuStel:

naH jajmeywIj bIQ'a' HeHDaq jIyIt
In my youth I walked on the beach.
My intention here is to imply that I used to walk on the beach;
it was my habit to walk on the beach in my youth

If this sentence describes a habit taking place during youth, then
shouldn't the sentence be {qaStaHvIS naH jajmeywIj, bIQ'a' HeHDaq
jIyIt} ?

The way I would understand {naH jajmeywIj bIQ'a' HeHDaq jIyIt}, is
that "in my youth, once I walk on the beach".

qaStaHvIS isn't some magical invocation, and has no special status in grammar. -vIS just means that while one thing is happening, another thing happens. If I had said, jIQuptaHvIS bIQ'a' HeHDaq jIyIt, you'd have no trouble seeing that as When I was young I would walk on the beach, as if it's a habit. Why should While such-and-such period of time occurs be any different?

You can use the qaStaHvIS, but you don't need to.

The sentence could be interpreted as In my youth I walk on the beach, but only if you're conceptually casting yourself back in time to those heady vegetable days. Ah, it's like I'm there now. It's my youth. I wake up and smell the sea-air. I take a walk along the beach. A crab suddenly takes a bite out of my foot. You've put yourself in the moment, and you're relating it as if you're there right now. As you state each of those actions, you haven't finished them.

As I said, how you interpret an aspectless verb depends strongly on context.

SuStel:

If I said
wa' ben Ha'DIbaH vISopbe'pu'
I didn't eat the meat last year.
This would be referring to some instance
in which I was offered meat and refused it.

I understand this; but is there any particular reason you chose to
write {vISopbe'pu'} instead of {vISoppu'be'} ?

No particular reason. I believe they are synonymous.

-- 
SuStel
http://trimboli.name