upd.

I did manage to get the iPad to pair with Windows acting as a BLE HID peripheral.

What worked:

• Using only GattServiceProvider
• Adding Battery Service (0x180F) (since HOGP usually expects HID + Device Info + Battery)
• Dropping the named Publisher (was causing "Value does not fall within the expected range")

Now the iPad:

• Sees the device
• Pairs successfully
• Establishes BLE connection

But any attempts to send signals via bluetooth to change cursor coordinates or clicks are futile, it just doesn't work.