As the name indicates it, there is quite a difference between both process
Editing requires a very thorough check of the target text against the original source document to ensure that nothing was missing, that the translation is accurate and does not differ from the original text. Editing normally may involve some rewriting and adjustments to fix stylistic and other problems as well.
Proofreading is normally a much faster process and in most cases it shouldn’t be necessary to go back to the source language. Basically involves minor checks and adjustments.
We always do a through QA process before sending out a translation to our clients
Wow, those must be rich clients. If I were to edit something, as you described it, I think I’d want practically the same price as a full translation. The way I usually do it is if a proofreader understands the source language they can refer to it when something seems odd. Otherwise I like to believe I’ve chosen quality enough translators to be able to trust them. Rather than hire a second person to comb through everything so meticulously. This sounds like some sort of ISO bureaucracy that only bloats the end price significantly. After the proofreader I like to have a vetting stage, which is to make sure that nothing is missing and that the formatting is correct. Much cheaper than the proofread.
Anyway, I understand ISO requirements and corporate thinking, and I’m flexible.