Hi Imbatronics
I got two of these CH346g MEGA2560s off eBay recently. I am not on Mac but I think something similar to what you suggest. Windows7 has problems with these devices, as does my KALI Linux and Windows XP. But suprisingly the 1.01 Arduino IDE on Raspberry Pi 2 recognised as a /dev/ttyUSBn. However, attempts to load code resulting in the sync from avr_dude.
Anyhow, after finding this thread, and some more digging, I found I could use my 1280 to check and program a boatloader for these chips.
See http://www.gammon.com.au/forum/?id=11635 for Nick's brilliant programs.
git clone onto your box
Run his "fixup" shell script
Wire up the 6 pin ICSP, recalling to put pin 5 on target ICSP to pin 10 on the mega1280.
Run his boot verification
Complete rubbish on the chip.
Load his bootloader onto it (this verifies the bootloader). And then one can load a sketch onto the Mega 2560 using IDE and it works!!!
But, and I think this is related to your observations, only once!
Try to load another program and it fails with the sync.
I reattached to the 1280 and used Nick's program to verify. Bootloader is good!
But wont sync.
Reloading the 2560 will work again -- then stop.
So I have a potential workaround.
1 - keep the 1280 as the "rescue me" for the 2560s
2 - develop my code on the 1280 until I am ready to commit
3 - using the rescued 2560, load the 1280 code onto it - hoping it loads first time.
goto 1 if I want to load another sketch on it.
I have written to the eBay folks I bought it from. Depending on their answer (and more research based on your hints) either I will get it to work better, or accept my workaround, learn that cheap is not always best, given them a poor survey, expect the rant this will entail, and move on -- RPi is probably where I want to be anyhow,
Regards
Steve H