Forget your story for a moment and revel in how this loss makes you feel. A loss of treasured words is a pain which every writer eventually encounters. It is agony, but it is also an opportunity. In this moment, while sadness, anger and self-reproach are burning within you, put pen to paper and capture how you feel. Use first person perspective and go deep. Write from the gut.
When the fires cool, look over what you've created and compare it to your regular style. There will be moments in future stories, when you want to make your readers feel like you feel right now. This is your opportunity to learn how to do it.
Now back to your question...
Unless you normally write against a detailed outline, I would discourage using a "new-to-you" outlining technique to give your rewrite efforts structure. Plotting/Outlining writers develop a specific set of writing skills which help them follow pre-existing writing plans. Free-form writers develop an equal yet different set of skills. If outlines are not part of your normal way of writing, then trying to use them now, while you are upset over the lost work, could seriously disrupt your writing flow and diminish (or even jeopardize) the final work.
You have successfully created 700+ pages using your current preferred writing techniques. Whatever those techniques are, they are working for you. Just Keep using them.
Read through the last few pages of your most recent backup, to remind yourself of where you need to pick up the story. Then get back to writing from that plot point. Where you go from there may not be identical to what you created earlier, but there is no reason to believe it will suffer from the rewriting. In all likelihood, your new words will affect the story deeply, enriching and enhancing it in currently unexpected ways.
There is nothing wrong with rewriting a hundred pages. That is an event which happens regularly during the edit phase of any novel's gestation. Replacing words is part of what we do. New words are always waiting to join the page. Have faith in them, that they will serve your story as well as their fallen comrades did before.