Thank you for your post and the video! Thankfully we are aware that if you place a weapon in your offhand the UI indicates that you are dual wielding and you cannot equip a shield in the main hand either. I'm unsure of how the designers will plan to change this as it could go a few ways, but the bug is reported. I will also add it to the known issues list for everyone here to track it as well.
As for the second issue, in your video it appears that you open the inventory before combat has actually ended. This is why you cannot change your equipment around at that time. You would have needed to close the inventory and allow combat to properly end to do so.
Thanks again for your support!