However, lets return to the {-chuqmoH}. We have the canon sentence
{Qo'noS tuqmey muvchuqmoH qeylIS}.

As you wrote: "There are two ways to interpret muvchuqmoH:
[muvchuq]moH he (singular subject) causes them (plural object) to join
each other and [muv]chuq[moH] they (plural, reflexive subject) cause
each other to join."

So, if I understand correctly what has been explained so far, then the
following two sentences are correct:

{verengan Duj vIHIvchuqmoH jIH}
I caused the ferengi ships to attack each other

{HIvchuqmoH verengan Duj}
the ferengi ships caused each other to attack each other

That last one is the Ferengi ships cause each other to attack or the Ferengi ship causes someone unspecified to attack each other. In neither case does the subject attack itself.

But if I wrote:

{verengan Duj luHIvchuqmoH cha' Duj nov}
two alien ships caused each other to attack the ferengi ship

{verengan Duj DIHIvchuqmoH maH tlhIH je}
We and you caused each other to attack the ferengi ships

Then both of these sentences are wrong.

Is my understanding correct ?

Yes. Unless Klingon allows reflexive verbs to also take objects, which there is, I think, slight evidence of. For example, the thinking goes that you can't say tuqlIj qun wIja'chuq.

-- 
SuStel
http://trimboli.name