OK, several hours later, I have a viable solution (see result in attached screen-shot):
1. reinstalled Hyper-V (because I need it) but did not create an "External Switch" for the VM's that need Internet access
2. checked that Wifi Direct still works (so not the installation of Hyper-V disables WiFi Direct but the "External Switch" that bridges to the WiFi adapter and only one adapter may bridge to it (its written in a popup when you try to add another adapter)
3. created instead an "Internal Switch". This creates a virtual adapter on the host that is connected to this new switch. No bridging of any type and no modification of the former Wifi adapter's behavior. This switch lets all VM's communicate among them AND the Host, which is connected to the switch as stated before. But no IP schema here. So I decided to do ICS. By sharing the Internet Connection of the WiFi adapter, it becomes a NAT, DHCP and DNS Proxy for the network where it is attached. So the VM's receive their IP configuration from it.
Now everything works fine.
I'll show it to my grandma so she can setup hers :-(
Windows is still no "Consumer" product. Or maybe yes, it's made for being sold. Some may end up by use it.
View attachment 7105