Nothing to be concerned about - that's well within normal operating temperature range for lithium batteries. Cases definitely insulate they will case your phone to hotter, doing so faster and staying there longer, than running without a case. 20 degree difference is surprising though, if both screenshots were taken under similar use.
I recently had the same question. You need to download the Android Central forum application. Works for me thanks to a forum member for pointing me in that direction. Only problem I have is I'm unable to log in to the forum application.