Suppose I write:

yaS HoHpu' bombogh yan 'ej reH vuQbogh

the officer was killed by the singing sword which always fascinates

Or

yaS HoHpu' bombogh yan 'ej ghIq ngabbogh

the officer was killed by the sword which sings and subsequently vanishes

I can understand and "feel" why in the {rIQchoHmoHbogh nuH ('ej} vaj 'oy'choHmoHbogh} the {vaj 'oy'choHmoHbogh} is a parenthetical miniature sentence.

But in the singing sword examples I can't feel anything parenthetical in the second {-bogh}.

So I guess the question is this:

In the case of sentences as the ones already mentioned, is the second {-bogh} phrase to be always understood as a parenthetical phrase? Or the distinction between what is parenthetical and what isn't, is rather based on context instead of whether it's being preceded by an adverb?

Eh? Neither relative clause is parenthetical. Relative clauses appear to be restrictive in Klingon — that is, the meaning of the sentence changes if you drop them.

This doesn't mean "The sword killed the officer (oh, and by the way, the sword also sings)." It means "The sword that is the one that sings killed the officer." The relative clause bombogh is restrictive.

This doesn't change just because we conjoin relative clauses.

yaS HoHpu' bombogh yan 'ej reH ngabbogh
the singing sword that always disappears killed the officer

This is literally "The sword which sings and which always disappears killed the officer." This doesn't mean "The sword killed the officer (oh, and by the way, it sings and always disappears)." It doesn't even mean "The sword that sings killed the officer (oh, and by the way, the sword always disappears)." It means that the sword that sings and always disappears, that particular sword, killed the officer. It's not talking about any other sword in that sentence. Both relative clauses are restrictive.

The problem you're having is with that ngIq. That adverbial is trying to pull the disappearing action into a period of time after the sword killed the officer, but you're also trying to use the relative clause to describe the sword that kills the officer. There's nothing ungrammatical about doing this, but you're confusing yourself because you're trying to imagine the sword that disappears in the future being used to kill someone in the past. The ngIq is meant to refer back to the killing, but it's conjoined with the singing. One would be quite justified in reading it this way:

yaS HoHpu' bombogh yan 'ej ngIq ngabbogh
the singing and then disappearing sword killed the officer

That is, even though it's obviously not the intention, this would be more naturally understood as a sword that sings and then disappears being used to kill an officer, not as a sword that sings... and then disapears after killing the officer.

When you come across problems like this, where what you're saying is grammatical but which is likely to be interpreted in a way different than what you mean, you need to rephrase. Not because you can't say it that way but because you shouldn't.

yaS HoHpu' bombogh yan. ngIq ngabpu'.
The singing sword killed the officer. Then it disappeared.

-- 
SuStel
http://trimboli.name