Hi,
I'd be interested to know if your Macbook can connect to the SL24R III via CAT5 just so we can confirm if you can connect at all?
If your Macbook has Thunderbolt, you should be able to fairly easily create an AVB connection between your SL24RIII and your computer which will allow you to do your multitrack recording (assuming the mixer is working correctly).
Obviously this doesn't solve the USB issue but it might be a temorary way around it to get your recording done.