yes you are welcome to use the lounge...
also, sometimes when you check in early in the morning, your room is ready....so don't assume that it won't be..
in fact, you should let the itinerary planning office (IPO) at the hotel you're staying at know what your arrival time is...
they might be able to have your room ready for you (if the hotel isn't too full)...
we didn't arrive early in the morning, only at noon....but when i spoke with the GF IPO, he asked what time we would be arriving....i said at noon....he said "no problem, we'll have the room ready for you"......he already knew at that point that it was doable....