I must not be understanding your question, then. You don't do any manual insertions with the Glyph menu. Unless you are the Khmer editor and you say "Oh yeah, translator used the wrong tone mark here, how do I fix it in InDesign?" I don't think that is what you are asking; please do correct me if I'm wrong.
Assuming the text is correct from your Khmer translation provider, then the thing you have to do to make sure that it renders correctly is to use the correct formatting and the correct composer, and to know InDesign's limitations. At no point should you personally need to use the Glyphs menu to fix composition. Here's exactly what I would do if I were in your position:
1) I'd use File -> Place to insert Word files with my Khmer paragraphs into InDesign. (Again, copy/paste should probably work, but I don't trust the clipboard.)
2) I'd apply Paragraph Styles with Khmer MN font applied, Khmer language applied, the WRC turned on, and the amount of default leading increased:
3) I'd finish formatting my document, then I'd make a quick PDF and I'd probably send it back to my Khmer translator, because I know that InDesign can't be trusted to get the line breaks correct in Khmer.
As an aside for other future readers of this thread, having problems with Khmer layout in InDesign... I do have some clients with Khmer translators who know how to use zero-width spaces correctly, and so I don't have to necessarily run those through post-DTP linguistic QA for line-break purposes. But other translators are less willing to work in this way (I mean, how many times in one's life can one be told "Forces external to your culture are changing the way you type your language! Get ready to re-learn how to touch-type with a new keyboard layout! Again!"), and I just assume that translator review is Always Advisable.
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