How to synch audio on many sound channels the best way?

I'm struggling with how to deal with sound files together with video.
Something went wrong so the timecode didn't work on our Epic and we usually do not have scratch track. This means manually synching since we did slate.

I have 7 channels of sound in individual files
Movie media from Epic, Canon 5D. I have file creation time which is mostly accurate and would like to keep it.

The way I have been experimenting with is
1) Synch first the 7 sound channels into one synched sound file.
2) Select the video file and the new sound file (with 7 channels) and press Synch. Then I would manually drag the sound file until it synched with the slate.

Is this the best way of doing the synch? Should I skip the first step? It feels convenient to have all the sound channels in one file.

A problem I have is that in step 1) and 2) I loose the creation time and date on the file. Is there any way of avoiding this?

Thanks a lot for any help!
/Andreas