The eating of posts when you try to post is a real pain and one that I didn't think had gone away with the latest SMF forum software version.
For me, I felt that the preview before posting was usually the best option, less chance of losing it when posting. It was rare for me to have the post eaten when using preview even though I used preview at least once in compiling a post. I have used it a few times during this post and no issue.
Before (on Firefox) using the Back (previous) arrow didn't help as the page would auto refresh resulting in the loss of the content. However, for a couple of FF updates I have been able to use the Back arrow and see the content, using preview then allowed me to post (without loss).
Whilst I was having the trouble, I did find Form History Control add-on for firefox and that saves text content that you input for a period of time, days even, so you can retrieve what you have typed into text fields.
That said, we really shouldn't have to take these measures (or copy and paste) to ensure posts weren't lost.
Wishing your wife a speedy recovery.