Focus advice - 700d/T5i and Saturn/Jupiter conjunction

I'm struggling to get clear/sharp images and whilst there are other factors in play* I also realise I'm not sure which focus mode to use.

Normally I'm on Back button which has the advantage in any photography of the focus not changing when the shutter is pressed.  I've been able to set the 100-400 +1.4 extender to give a focus acquired beep with the planets in the single shot centre point focus modes selected.  However when I had my 450D I found I could be more accurate by going to Live View.

Now Live view seems to have a mind of its own and wants to focus whatever is in the frame.  I can set to Manual focus of course and turn the ring, using a magnified view, but the mere action of touching the ring causes a wobble and the planets are moving anyway across the screen.

I am using remote shutter releases.

So I'm looking to settle on a technique to focus accurately on the planets in the hope that I can get a sharp photo.  Note that with the 450D and 55-250 I have captured Jupiter's moons and I expect to be able to do better with the 700D and 100-400 IS II

* 1) I'm using a Kenko x1.4 extender, I suspect that this is not improving my IQ.

* 2) Here in the UK the (near) conjuction is low in the early evening sky and there's a lot of moisture in the southerly winds neither of which are conducive to sharp clear images.

You might want to also ask this in the Astrophotography forum, there are several threads about the conjunction already.

I dabble with the moon sometime with my 55-250STM, but planets are beyond reach for me.

IMHO manual focus would work best, but I could be wrong.

Here in Atlanta it will be cloudy tonight, besides today is my wife's birthday and that would take precedence anyway.

Glenn

EDIT: Actually, the clouds were yesterday's forecast, now they are saying 4% sky cover tonight, maybe I can take a glimpse at it.

I asked here because the focussing questions are very Canon specific especially Live View operation which is one of the regularly recommended methods in Astronomy forums.

Ray UK wrote:

Use manual focus in live view at maximum magnification. Set the focus by pointing at the moon, once focused do not alter it just reposition the camera to point at the planets.

I don't have any problem with PDAF focus on the moon at 400mm but am surprised that it would be sufficient then for to lock that focus and use it for the planets.  Just not enough clear nights for experimentation and then the whole thing will disappear below the horizon.