Hello, I do not have the dual-boot ROM installed and cannot check the issues you have with Bluetooth KB and mouse. But Linux on these types of devices will usually work with this sort of thing if keyboard and mouse use a USB Bluetooth dongle.
There are two extra USB ports on the Vim's GPIO, see section on VimUstension for an easy way to access these.
Yes, usually if an Android TV box has builtin WiFi module, external WiFi adapters won't be available. A very few(I know of only 1) makers have made a setting in Android to select the module used and can use USB attached external WiFi adapters. Maybe Khadas can add this feature at some time.
If you are looking to increase WiFi range, the Vim's antenna is connected to the PCB via an IPEX connector, adding an external antenna might help and should be easy to do(no soldering req'd).
There is an inexpensive external antenna set available for the GeekBox that comes with two antenna, though you would only use one. I am not sure if the connector is the same on the Vim, as it is on the Geekbox, this would need to be known beforehand. If not, other Antenna options exist...search Google or sites like Amazon, ebay, Aliexpress, etc.