Hi Peter,
I have a Samsung S3 and can record full HD video on it, the video quality is impressive. The sound quality is not bad but leaves a bit to be desired unfortunately and I find I need to take it into my PC for tweaking.
Do you have anywhere you can edit video, i.e. a PC or Mac? And a program to edit video?
You could record video, (and by default the room audio) onto the S3 whilst simultaneously recording the audio from the guitar straight onto the Zoom. Import both the video from the S3 and the audio recording from your Zoom (presuming that you can transfer the audio file from the Zoom into a computer, USB?).
In a video editor, strip out the original audio from the S3 video recording and substitute in the audio file from your Zoom. It will involve a slight bit of aligning the audio to the video to get them into sync.
Then export in a video format suitable for You Tube.
Easy peasy eh! Until you actually try it and it can be a bit fiddly to say the least

It took me a while to find a working method for producing You Tube videos but you only have to crack it once and you can then make as many videos as you want
Alternatively just record it on your S3 and leave the audio as is, it may not be as good but certainly easier to achieve
Guitar looks great BTW, wish it was mine

Edit - just noticed that you have an Imac, never edited video in an Imac but I believe it's set up to be easy to do, is it easy to get the files into an Imac? Just read that the latest Imac has a 720p camera built in with noise cancelling microphones, would it be an option to record straight into your Imac?