I was there in February... Decent food, but saw no showers... If you are just transiting EZE, if you exit security and come back in, you'll have to pay the $18 departure fee (at least I did a year ago when I had to do that)...
If AC is using the lounge, I would presume that it would open earlier (the AC flight is at 3:45 pm 5 days a week). In the past (2007) I arrived at 11 am and was able to use the lounge until evening departure...
Good luck!