Long experience has taught many of us that when you send a busy person an email with more than one question, they only answer the first one and ignore the rest. Thus many of us have gotten into the habit of asking one question per email. You may be able to solve part of the problem by asking writers to send you a bunch of questions in a single email -- as long as you actually answer them all.
But don't expect that the first thing you are going to get is a completed document with review questions attached. Technical writers seldom have access to sources of information that would allow them to write a complete document without asking engineers a bunch of questions. They need your answers to get the document written.
As to FrameMaker: Framemaker comes in two variants, unstructured and structured, and these days structured Framemaker come with support for a structured writing system called DITA. If your writers are using the structured or DITA versions of Framemaker, then yes, there is an XML version of the document that can be reviewed with standard code-review tools. If they are using unstructured Framemaker, however, the answer is no. My impression is that most organizations who use FrameMaker are still using the unstructured version.
A common method for doing review of documentation generated with unstructured FrameMaker is to use PDFs and use the review tools built into Adobe Acrobat reader. There is even a server-based version (at least I assume it still exists) which lets multiple reviewers comment on the same copy of a PDF so that they can see the comments that others have already made.
Personally, I would rather have a root canal than do review this way. Like you, I would prefer that the whole process take place using structured text and standard text-based review tools. But many technical writers seem to prefer this approach, and there are also some engineers who insist that they have to review the final printed form of the document -- I have no idea why, but it is an unfortunate fact of life.