The verb {chep} means "to be prosperous, to prosper", and so we have
the {yIchep} for "prosper !" (imperative).

But if the only definition given to {chep} was "to be prosperous",
then we couldn't have the {yIchep} and so we would have
{yIchep'eghmoH}, right ?

Yes, yIchep violates KGT's rule that imperative verbs expressing states or qualities just include -'egh and -moH. The rule has been violated elsewhere too, mostly but not always before KGT was published.

..and with regards to the {Dab} on the paq'batlh example.

A little bird told me, that according to the KLI mailing list
1999.07.19, "In Klingon, when one lives at a place, he or she is
thought of as occupying or inhabiting it. That is, he or she is not
seen as doing something at a location, but rather as doing something
to it"

So in this context, the {'op ben pa' Dab ngan} should be written
instead {'op ben pa' luDab ngan} for "some years ago inhabitants
resided the there".

The only interpretation of the canon sentence which I could find,
in-keeping with the above list's comment, is "some years ago an
inhabitant resided the there". But I don't think this is the intended
meaning; if only one resides somewhere, then this somewhere hardly
qualifies as being alive and prosperous.

I can think of three possibilities:

1. It was supposed to be luDab, and the lu- was erroneously dropped.
2. The subject is singular: some years ago an inhabitant inhabited there. This is not an unreasonable way to render literally the figurative [the land] was alive, though I would have expected plural inhabitants.
   
3. It is using pa' as a non-subject, non-object noun placed before the OVS structure, and Dab here has no object: some years ago, thereabouts, inhabitants inhabited (in general). This is not actually ungrammatical, just a bit odd.
   

-- 
SuStel
http://trimboli.name