It's sometimes tricky to find the right timing. Try pressing the left button for 10 seconds, then press the right button until it changes into Engineering Mode. Or 7 seconds. It's something between 5 and 10 seconds and it always works (at least with US sw up to 0110 and Euro sw up to 0360).
Doesn't matter. The only thing I found was that it seems to be more difficult once you left Engineering Mode and try it again. Seems to be easier to do it after the initial power on. But that might be me.
I agree. Make sure you hold down the left button more than 5 seconds, I used to always try for exactly 5 seconds which is not necessary. It works almost every time for me now.
Also, you don't have to be lightning fast between button presses. By this I mean after you press the NAV button to start the sequence, wait for the nav screen to show up... sometimes it takes a second after the unit is first turned on.