Thank you Dan & shoppingamc. I doubt many people are still upgrading mid 2010 MacBooks. But your feedback is appreciated and hopefully this may help someone else.
Your feedback has helped me accept that I must in fact have a machine in that subset of a subset that can’t upgrade to 16GB. Besides being stuck with an extra 8GB RAM chip, I have been very happy with the upgrade. And I have come to understand a few things I’d like to share.
One point of view is that 16GB RAM is overkill in this machine and would present an undue/unnecessary burden on the battery anyway. And while it is said you should not mismatch your RAM, I found the reason for this is that mismatching your RAM will generally reduce clock speed/performance by about 10%. But since I’d only have 8GB max with matched RAM, I should be getting better performance from my mismatched 10GB RAM chipset than an 8GB RAM setup that is matched (4&4).
So I will also just be happy with my 10GB of RAM. Perhaps down the road I’ll install Mojave and see if the Mojave upgrade “force pushes” the EFI/firmware to allow the full 16GB RAM. But since by any metric the upgrade has been a huge success it will not be until a time when Apple stops support for High Sierra and am more or less forced to install Mojave.
I’m scared installing Mojave will cause other issues not worth it since the machine is now performing so well. I haven’t seen this recommended anywhere and it’s just my hunch this may work. I am very happy with the results as is and think I’ll just leave well enough alone for now…