Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
For what it is worth, when I took Sushma's original suggestion and posted to the Office 365 forum here: https://community.office365.com/en-us/f/156/t/412487, it began another thread just like this one--accompanied by a MS support response that dodged the question by pointing to which products require which types (personal/corporate) MS accounts. Not a word of which products, if any, might allow activation by product key only.
For sure, buying retail (if even that can be activated w/o an account in 2016 versions as it was in 2013) will be cheaper than OEM, at least from my time-lost perspective (i.e. considering all the extra wasted time I spend trying to figure out and babysit the OEM licensing process now). Of course, I cannot charge my clients for my headaches or extra time involved, but I will charge them the extra for retail. The loss of time for me is just too significant for me to risk repeating this beating-head-against-wall over OEM licensing!
In last night's debacle, I finally just gave up on OEM 2016, bought a retail copy of 2013 H & B online from a retailer that still carries it, and this activated w/o a MS account. I guess I will just keep the 2016 for when I get a new computer and can put it on my own MS account. But that is not something I can repeat regularly.