Suppose I write:

< Duv > jatlhpu' 'e' DaSov

you know that he said "advance"

Since there's no rule prohibiting placing a quotation before the {'e'} of a sao (as long as the verb after the {'e'} isn't a verb of speech), seemingly/apparently this sentence would be correct.

Be careful. What you're doing isn't placing a quotation before the 'e'; you're treating a sentence-as-object construction (the verb of saying and its quotation) as the first sentence of another sentence-as-object construction. You're nesting SAOs.

The structure is not this:

<quote> < <jatlhpu'> 'e' DaSov>

The structure is this:

< <quote> jatlhpu' > <'e' DaSov>

You could also say it like this:

< jatlhpu' <quote> > <'e' DaSov>

So, similarly, wouldn't the following be correct too?

< HeD > jatlhpu'; < HIv > jatlhpu' 'e' qa'

instead of saying "attack" he said "retreat"

Yes, I would accept that. The fact that the "replacement" is a sentence-as-object instead of a basic sentence appears to be of no import.

Of course, one could argue that the {'e'} of the {'e' qa'} isn't the "classic" {'e'} of a sao, but if that's the case, then even better, right?

It is the classic 'e' of an SAO. When you're saying

<sentence1>; <sentence2> 'e' qa'

You need the 'e', because it literally means it replaces that <sentence2>. It's only when you're instead-ing noun phrases that 'e' is not required.

'awje' vItlhutlh; HIq qa'
'awje' vItlhutlh; HIq 'e' qa'
I drink root beer instead of alcohol.

Technically, the 'e' in the above sentence is ungrammatical, but Klingons may say it anyway. But when dealing with full sentences, the 'e' is required and is the same old 'e' we've always known.

jIQam; jIba' 'e' qa'
I stand instead of sitting.

This is literally I stand; it replaces that I sit. You cannot say jIQam; jIba' qa', because the object of qa' replace cannot directly be a sentence. You need the pronoun to stand in for it.

-- 
SuStel
http://trimboli.name