You should try the Wi-Fi connection. Media Streaming doesn't actually take up that much bandwidth. You may see no effect on your laptop at all. If your TV has the LG Wireless module it supports IEEE802.11a/b/g/n by the look. Plenty fast.
All the scaremongering about Wi-Fi EMS radiation is just that. Scaremongering. Ignore it. If Wi-Fi is on, you are getting the full dose anyway. Power management may turn down the Wi-Fi power, but if you are using it, it is probably running at full power... which still isn't an issue for us humans, or our plants, cats, fish etc.
Windows 10 allows a great deal of configuration, even allowing us to set up stuff that we never should. Just because you can, doesn't mean you should.
If you insist on your Ad Hoc network solution, setting a Static IP Address on the TV and the laptop ethernet should work, and allow connection without further effort by you. Leaving the TV set to 'automatic' IP mode will always be slower and problematic. I mentioned using a Static IP Address on the TV earlier, but it sounds like you have tested lots of Static IP Address configurations.
So if you had something like:
Laptop 192.168.0.10 255.255.255.0
TV 192.168.0.20 255.255.255.0
Then the connection should work without further action by you. But it may be slow to connect. Certainly slower than establishing a normal network connection, whether it is Wi-Fi or ethernet.
I missed this:
You seem to say that, unlike Vista, Win10 does not allow these two networks, WiFi & LAN, to co-exist on a device, except on 'Ad hoc' basis where static IPs aren't needed (all done by DHCP)... Is that what you mean?
No.
DHCP is used for automatic IP Address assignment, not in Ad Hoc networks.
An Ad Hoc network can use Static IP Addresses assigned by the user, or Windows automatic private network IP Address assignment, which is slow and I suspect limited in some functions.
Windows 10 will allow the two networks connected to the two separate network adapters. But that may have adverse consequences on the laptop, which now has to manage two connections.