Hi all, got some positive developments.
This sunday I tried a last attempt as follow:
i) following Nick's suggestion I uninstalled everything (Mac dive, drivers, and everything related). For each uninstalling I rebooted my Mac (3 reboots).
ii) I installed again everything in the following order: a) Macdive; b) Prolifc driver; c) third party driver. Following each installation I reeboted the Mac (again 3 reboots);
iii) I to connected again everything BEFORE LAUNCHING Macdive and this time worked.
So I have been able to download on both Subsurface (as I succesfully did the previous day) and on Macdive.
What is interesting to notice is that now both Macdive and Subsurface see a different port: named /dev/tty.usb1004. However, in order to succesfully download dives I have to (manually) select the port named /dev/tty.usb2006 which was already present before but didnt work at all. Macdive downloaded and numerated almost all dives (2014 correctly numerated in their sequence number, while 2015 dives have a ID number equal to 0). Please find attached a screenshot.
To sum up: it works but I dont really know why and how

. I dont event know if the uninstalling/rebooting/reinstallling/rebooting thing has anythig to do as I did it before and it didnt work. But the most important thing for me is that now it works!
I really wish to thank Jeff and Nick and everyone at Macdive which helped me out of this mess and guided me in doing tests and other stuff that I am not familiar with.
Now looking forward to my next dive!!!