I do not know Lincoln personally as I do not live there, but I can tell you some of the general things you said.
Across the Downs. I've already said on this forum that I detest the Downs, especially in the winter due to the wind that blows down them and also the rain on them, they are not a pleasant place to be-however, as I got told, you can use the hopper bus, but this'll involve you waiting for it in the bus stop, by which time you may as well have just braved it!
JCR. Most (if not every) halls have a JCR and it is up to the discretion of the warden as to how long you are allowed to stay in there. In mine, it was 24hrs a day, then some people let rubbish in there and whatnot, so it has been shortened to, what I believe to be, 11pm, as with Lincoln. Most halls also have a bar yes, and this is open until 11 on a normal night, and some nights they do stay open for longer, for instance after formals if the afterparty is to be held in hall. But, I must say that I find that the majority of people do not use their JCR, only a select group of people use it. Others tend to hang out in people's rooms or the like, and only tend to use the bar as a meeting place, or for a few pre-drinks before going into town-yet people's rooms are often used for this too. When you can go to a supermarket and get drink far cheaper than the bar, or go to a bar in town, say BZR triples-for-singles, the hall bar does not look so good.
That's just my personal opinion on it anyway.