I wouldn't worry too much about it - as I doubt that there is anything you can do but wait.
One of my sites is in an identical situation. The home page is showing an old cache date, whereas all internal pages are showing a recent cache date. This all happened when this "update/rollback/tweak" began last week.
I know for a fact that G HAS indexed/cached the home page with a more recent date than the one it currently shows in the SERPS, so G DOES know about it.
Once this situation (whatever it is) is complete, I would expect G to be showing a more recent cached date for the home page too.
I believe the best natural way to get GoogleBot to crawl your site consistently is to have quality links coming to your site as well as quality content being added to your site. If you were to add a page of content a day, I could assure you that the spiders would be on your site everyday. If you have not been crawled in a while, I would recommend you start by a link from a site that is consistently crawled. Then it is up to you to add the quality content.