Will the EF-42 help the X100 focus under party/nightclub conditions?

Not sure I can respond to your specific questions regarding the gear and focusing, but you may find the images and technique of this contributor (Hos), who was once active in this forum, of interest.

If you check out the images in this thread (http://forums.dpreview.com/forums/post/37361176) and some of his others too, you will see he uses flash to isolate and 'freeze' the prime subjects while balancing it up with the ambient light, capturing the atmosphere of these types of events with motion blur and non-balanced colour as a background.

He seems to have moved on to other gear but he posted quite a few threads of 'partying' events and has a website that you can access via his profile.

I found his work unusual and inspirational. Perhaps his technique might be of interest.

Norman Young
www.noyo.co.uk