1. I can see d’autant plus + adverb + que, but not d’autant plus + adverb + pour. Could you give an example with pour that we could look at?

2. I don’t think a priori that this en has to do with a pour-phrase or que-clause. I think it only stands in for Sa voix restait si calme et naturelle.

3. Yes.

Your third option seems all right.

Sa voix restait si calme et naturelle que mon exclamation m’en parut d’autant plus ridiculement emphatique.

Without changing the whole structure, you could reconstruct a de-headed complement:

Sa voix restait si calme et naturelle que mon exclamation me parut de ce fait d’autant plus ridiculement emphatique.

But of course, you could also reverse subordination, getting a completely different structure for approximately the same meaning:

Mon exclamation me parut d’autant plus ridiculement emphatique que sa voix restait si calme et naturelle.