I find it surprising that you wouldn't need to customize it at all to emphasize which points are most relevant to the question you're answering and explain how they apply. Many English questions might be answered by quoting the appropriate pages from either of the CGELs, but that's not really the most helpful way to answer a specific question. If you don't feel comfortable writing an answer from your own understanding supported by that source, maybe leave that question for someone else to answer and just comment with a pointer toward the other material.
Telling someone “Read this text from properly cited source. Here is a copy of it for your convenience. Any questions?” isn’t as helpful as trying to understand why someone asked the question and tailoring your response to address their specific source of confusion. I think there is no such thing as “reinventing the wheel” when teaching. Would a good teacher hand out a textbook at the start of a course and just say “Here, read this. It’s a really good explanation.”?
I sympathize with wanting to use a source that has been looked over by an editor and had the grammar and spelling corrected, but on Stack Exchange sites you have an entire community of editors willing to help correct any mistakes. The site is set up so that posts can be improved over time. Someone who couldn't answer the question because they didn't understand the topic as well as you do can still contribute by correcting any mistakes you might have made in your answer. The community works together to make the content better.
Your perspective and expertise is more valuable to the asker than your answer being written in perfect English. Even just explaining why you think the source you've referenced in your answer is good is useful to someone who may not know enough about the topic to be able to judge which sources are credible.