I have 3 LG G3 Verizon phones and using Tracfone as the carrier. One updated to Marshmallow - 2 will not update. Have you been able to update your phone?
Possible that overheating complaints are caused by the plastic,leather aftermarket phone protectors everyone is using? My LG G3 gets pretty warm if I'm charging and using at the same time - never enough to throttle, or at least that I notice.