Replacing main audio in iMovie 10

What I want to do sounds pretty simple yet neither tutorials from Apple to Lynda.com nor various Youtube clips seem to cover this!

Here is the set-up: shooting with a Digital SLR and external audio recorder. All files correctly imported etc. I then align the external audio with the original audio and want to replace one with the other so that I can then edit and trim as I see fit.

1) If I turn the volume down on the main track and then trim the clip, the new audio isn't locked: in other words I would need to edit all the video first and only at the end replace the audio. This is virtually impossible as I need the 'clap' sound to visually synchronise the audio but the minute I start trimming I loose the synch (aaaarggghh!)

2) Option 2, I tried to detach the original audio (easily done) and then reattach the new audio. Can't seem to be done: you're left with an empty 'blue track' and a linked 'green track'. Re-aaarggghh!

3) Finally – and as an add-on question – I find that synching sound visually is quite crude, I can't seem to get it perfectly right. Is there a way to move either the audio or the video tracks on the timeline frame by frame? I seem to remember that in older version Control Arrow left or right achieved that. Is there a modern equivalent?

Thanks for helping me preserve whatever hair I have left!