When we say {DaSjaj mavum}, the {qaStaHvIS} is unnecessary. Why ? For
two reasons: First, because there is no direct need to specify that
during the entire day we will be working.

qaStaHvIS DaSjaj mavum doesn't say you're working for an entire Monday. It says we work during Monday. We might work for one minute or one hour or eight hours or twenty-four hours.

if we are answering to the question:
"on monday do you want to grab a coffee?" then we can answer {ghobe',
qaStaHvIS DaSjaj mavum}. It is not that the {ghobe', DaSjaj mavum} is
wrong; it is only that it doesn't carry the punch of saying "no,
during monday we work".

DaSjaj mavum would be used to point to a calendar to show why we can't go for coffee that day. qaStaHvIS DaSjaj mavum would be used to set the context that Monday is ongoing, and then work happened.

But when we say {qaStaHvIS wa'maH DIS maSuv} for "during ten years we
are fighting", the {qaStaHvIS} is necessary because we obviously want
to convey, that the event of "our fighting" takes place over the
period of a large time span.

Since we want to convey the "during"
aspect, then obviously the {qaStaHvIS} is necessary. If we didn't use
it, then perhaps the reader/listener could be left to wonder: "did
they fight during the entire time-span ?".

Without it the reader/listener would be left wondering what that wa'maH DIS was doing in the sentence. Is it an object? What's its grammatical role? wa'maH DIS does not mean for ten years; that for is the qaStaHvIS. wa'maH DIS means only ten years.

Now, perhaps there is an additional reason.. if we just wrote: {wa'maH
DIS maSuv}, then the feeling that I get from this sentence is that we
have a {wa'maH DIS} which is very "independent/out of the
blue/undetermined/undescribed". And perhaps, here is the problem with
big periods of time requiring the {qaStaHvIS}; one can understand if
we just say {po'}, {DaSjaj}. {DaSjaj, povjaj je}. These are small time
spans.

Your feeling is correct but it's not because of the size of the time span. It's because a time span is not a time stamp.

And then we have the canon example of voragh:
{qaStaHvIS wa' ram loS SaD Hugh SIjlaH qetbogh loD}
4,000 throats may be cut in one night by a running man

Here the {qaStaHvIS} is essential in order to differentiate between an
event happening "during a night", and an event happening "one night".

Which works because wa' jaj one day works in the same way as it does in English: as a time stamp. We see this in the proverb wa' jaj 'etlh 'uchchoHlaH tlhIngan puqloD; jajvetlh loD nen moj. This is a time stamp. Don't think of wa' jaj as a time period, like cha' jaj or wej jaj, think of it as "one of those days I can point to on the calendar," or "this day, not that day."

However, a final question comes to mind: "could we have it as rule,
that when something happens during a small period of time then the
{qaStaHvIS} is always unnecessary" ?

-- 
SuStel
http://trimboli.name