Need to attach NTFS USB drive for videos to RPi2B with OSMC

Yes NTFS read/write is supported on OSMC.

Keep in mind that if you are trying to power the drive directly from the Pi 2 you will need to enable max_usb_current=1 in config.txt and have a good power adaptor. Not all drives can be successfully powered from a Pi so you may need an external power adaptor for the drive.

Yes to both of those.

To disable internet access the easiest way would be to use a Static IP configuration instead of DHCP and simply set the Default gateway deliberately to an IP address other than your router - eg an address not in use by any device.