Is it okay to begin a sentence with 'ach?
If I wanted to rebuttle someone in English, I might use the word "but" at the beginning of my sentence. Although it's incorrect, it is conversational. Can the same be done in Klingon?
I would say, that you have given a good explanation. 'ach is a type of sentence conjunction so to be fully grammatical it should connect two sentences into one sentence. However, it's not unusual to complete a sentence. Stop. Think a little more. And then decide to add a contradictory thought by starting with 'ach. I'm the type not to criticize it in either language, but others might.
It's not incorrect to use but at the beginning of a sentence. Some style guides and English teachers will tell you not to do it, but style guides and English teachers do not describe how people REALLY speak; they prescribe rules to meet their own standards of "good."
The question, anyway, is not whether you can start a Klingon sentence with 'ach because you can start an English sentence with but. The question is whether Klingons start sentences with 'ach. The answer is yes: in Star Trek V, Vixis has a line: 'ach HoD, Hevetlh wIghoSchugh veH tIn wI'el maH'e' But captain, that course will take us into the Barrier as well (literally but captain, if we follow that course WE will enter the big barrier).