Hi Justin,
Cutts has said more recently that there should not be any problem going the all https route, although he notes to try it out first on a 'lower traffic domain' if possible(!). https://news.ycombinator.com/item?id=4801641 Quite a few popular sites are heading this way (twitter, fb, paypal for example)
Assuming that is correct and you are not seeing any ranking/indexing issues with the site currently in https, there should not be an obvious reason to worry.
For your second question, again there should be no issue with running the blog on a non https url, but you would do well to avoid having links to the https version of the blog that are then 301 redirected to the http version. So making sure all your internal links to the blog are already http would be a good idea and ideally all links from the blog back to the site would already be https.
Beyond the page loading times, https urls can often have gotcha moments where certain elements of the page (js or css links, image urls etc) can be non https and produce warnings in some browsers which is bad for user experience. Sometimes the pages these errors occur on can be pretty deeply hidden so good to be methodical. Also need to keep in mind that your robots.txt and sitemap should also be served on the https protocol and make sure that your sitemap is not automatically generating https urls for all the site (blog included) which can potentially create unneeded confusion.
Hope that helps!