] .... For instance, had Lydia from P&P not been out, does that mean she would not have been allowed to "date" Denny or other soldiers (as in spend time on her own with them) or would it also mean that she wouldn't be allowed to attend the Meryton Assembly and Netherfield Ball? Or does it plainly mean that she can't interact much with men?

] Basically, all of the above. It's question that perplexed people living in JA's time, it seems. Mary Crawford spends a lot of time wondering whether Miss Price is out or not.

I get the feeling a girl who is older by still not out, could attend parties hosted in their own homes, but not attend ones hosted in other peoples.

at the request of her younger sisters, who, with some of the Lucases and two or three officers, joined eagerly in dancing at one end of the room.

These Lucases could be male but that seems like too many men. There must have been at least one younger lucas girl. And it says some of the lucases. And at least some of them might have been charlotte's younger sisters who were not yet out.