So, I managed to find my solution. args is a list of list, then each arg is a list (seems obvious now :p).
## This one works.
list( type="buttons", y = 0.3,
buttons = list(
list(method = "restyle",
args = list( list( y = list(myData$Y1), mode = list("lines"))),
label = "0 - Modify 2 attributes via restyle: y=Y1; mode=line [The solution]"),
list(method = "restyle",
args = list( list( y = list(myData$Y2), mode = list("markers"))),
label = "1 - Modify 2 attributes via restyle: y=Y2; mode=markers [The solution]")
)
),
Hoping it will save time and patience for others !