I was wondering if this was some kind of idiom, or a Klingon saying. "He will die amongst sarks and targs" sounds like the perfect Klingon insult, actually, the equivalent of calling someone a wimp. To die amongst (or perhaps even at the claws of) semi-domesticated animals? Not such an honorable death. :-)
Mostly adding ghaH to an otherwise already complete third person sentence does not effect the meaning. ghaH is a pronoun for a third person "being who is capable of language". It is usually translated into English as "he" or "she". Since the null prefix (the absence of a prefix) already tells us the subject is 3rd person, we usually don't need ghaH in the subject location. However, if I've been talking about a person and a thing (like a pet for instance), I might add it on just to be sure you realize I'm talking about the person and not the thing.
The prefix (or in this case lack of it) is not optional. If you leave a prefix off it will be read as third person. However, the pronouns are completely optional and mostly the sentence means the same thing with or without them.
I should add, however, that the pronouns can sometimes be used to add a little subtle emphasis:
qajey "I will defeat you."
qajey jIH "I (and not someone else) will defeat you." Particularly if someone has asked something like, "Who will defeat me?"
SoH qajey "I will defeat you (and not someone else)." Like a response to someone who says, "You have defeated everyone but me. What will you do now?"
This kind of emphasis is usually said with a little extra stress on the syllable in English and can sometimes be indicated in writing using bold or italics or all caps. In Klingon it's easier to add in writing and more fun to say in speech.
None; I believe both are intended to be singular in this sentence ("between a sark and a targ"), although they could be plural and mean in the sentence, for example, "between the sarks and the targs".
My understanding (and I stand to be corrected by the experts) is that the word sargh can mean "a sark" or "sarks", depending on context.
If you want to emphasise the fact that you are talking about more than one sark, you can say sarghmey. The mey is an optional plural marker.
Sometimes the verb prefix may change depending on whether sargh refers to one or many sarks. For example:
sargh wIlegh = "We see a sark." sargh DIlegh = "We see sarks."
Dulegh sargh = "The sark sees you." nIlegh sargh = "The sarks see you."
I have noticed that as a practical matter, the Klingon course here on Duolingo generally uses mey whenever a plural is intended. There are no mey's here, so you can assume that sargh is meant in the singular.