[LG Optimus L9 P760, P765, P768, P769] OperationNT:
Thank you for your answer. I am sorry, I made a mistake in my post : when I said "Volume Down", I meant "Volume Up" :( . When I manually reached the "S/W Update" screen, it was by doing the good action ;) .

Unfortunatly, even with "EMERGENCY" or "CS_EMERGENCY" mode, the result is still the same : the LG Update Tool refuses to perform a flash directly from the "S/W Update" screen. The output text is the following (in both "EMERGENCY" and "CS_EMERGENCY" mode) :

22:45:12 : [C:\LGMobileUpgrade] Try to delete folder
22:45:12 : RemoveDirectory1 Error(2):C:\LGMobileUpgrade
22:45:12 : Start Date : 2013-04-02 22:45:12
22:45:12 : ---------- QM Testmode ---------
22:45:12 : ***** Check Tool Version *****
22:45:12 : Tool version : 1.5.4.8
22:45:12 : UpgradeDLL version : 1.2.6.7
22:45:12 : Current Dir : C:\Documents and Settings\All Users\Application Data\LGMOBILEAX\B2C_Client
22:45:12 : LGE USB Driver Version : 4.9.8.1
22:45:12 : Android PDLV Version :
22:45:12 : Android NDLV Version : 3.8.0.0
22:45:12 : ******************************************
22:45:12 : [C:\Documents and Settings\All Users\Application Data\LGMOBILEAX\Phone] Try to delete folder
22:45:13 : SetEmerModeAT()
22:45:13 : _IsConnectedPhone Call
22:45:24 : _IsConnectedPhone(0)
22:45:24 : 0 - NotConnected.
22:45:24 : 1 - Normal connect success
22:45:24 : 2 - Emergency connect success
22:45:24 : *****CheckOS Start********
22:45:24 : Microsoft_Windows_XP_Professional_Service Pack 2
22:45:24 : *****CheckPCSyncPrograms Start*****
22:45:25 : PCSyncPrograms Not Found - OK and ready to upgrade
22:45:25 : Checking the connection with the cell phone...
22:45:25 : dwWaitResetTime(40000 ms)
22:45:26 : _IsConnectedPhone Call
22:45:35 : _IsConnectedPhone(0)
22:45:35 : 0 - NotConnected.
22:45:35 : 1 - Normal connect success
22:45:35 : 2 - Emergency connect success
22:45:36 : _IsConnectedPhone Call
22:45:45 : _IsConnectedPhone(0)
22:45:45 : 0 - NotConnected.
22:45:45 : 1 - Normal connect success
22:45:45 : 2 - Emergency connect success
22:45:46 : _IsConnectedPhone Call
22:45:55 : _IsConnectedPhone(0)
22:45:55 : 0 - NotConnected.
22:45:55 : 1 - Normal connect success
22:45:55 : 2 - Emergency connect success
22:45:55 : [009,400,102,102,102]The USB cable and your cellphone are not connected.\nPlease connect your phone and wait for connecting \nthe USB port for a while and press the [009,700,147,062,092]'Restart'[009,400,102,102,102] button.
22:46:05 : Check Cancel button active..

I tried to unplug and plug again the phone after 10 seconds (as it is advised in the unbrick totorial to force driver installation) : the result is still the same when I click on "Restart" from the update tool. In addition, I don't understand how it could be a driver issue : there is no unrecognized device on my Windows drivers manager and I am able to perform a flash as long as I start from a booted Android with the right options (the LG Update Tools restarts itself the phone in "S/W Update" and it works : the strange part is that during a more advanced update process step, while the phone is in "S/W Update" mode, my Windows pops up a found new device panel, but even if Windows doesn't find any valid driver, the update process continues and the result is right).

Do you have any other idea about this direct flash through "S/W Update" mode issue ?

Thank you in advance for your answers ;)