Mea culpa, I was misinformed. Because I tend to avoid using JavaScript to set inline style attribute values, I looked only at this CSS
I misread your post that you were suggesting adding user-select: text, instead of removing user-select: none (which indeed is being added by script). Hence I thought you were suggesting removing the spoiler feature.
Posts with quoted post content of course follow the original post. But if the spoiler was easily made visible, it could defeat the original members intent of spoilering.
A problem is the reply - quote post pulls in the raw, while the select copy to clipboard captures the rendered. AFAIK, clipboard can not be made to do otherwise.
As for the invalid HTML, the current cooked HTML is like
<p>not spoilered </p>
<div>
<p>spoiler text</p>
</div>
<p>not spoilered</p>
For some reason I was thinking (or more correctly not thinking 
) you wanted something like (pseudocode)
<spoiler> text
<p>quoted text</p>
more text </spoiler>
i.e. spoilering the quote, not quoting the spoiler.
Sorry for the confusion.
Have you tried forking the plugin with the script that adds the “none” removed?