Yes, the Halls of Mandos do sound like a pretty mournful and depressing place. The duration of the stay depends on the deeds done in life though - it is a kind of purgatory is it not? (I am not a catholic, so I am not certain what Tolkien modelled them on, if anything).
An elf like Feanor would likely spen a looooong time there, if not eternity. It is difficult imagining him ever repenting!
But say, for example, if Elrond was killed. I think his stay would be relatively short, since he does not appear to have committed any sins that we know of or can imagine! Except maybe pride (?).
I am not sure that the elves are necessarily separated from each other. I seem to recall someone learning from someone while in the Halls of Mandos, but I cant recall the exact reference of that.
"...Finwe grieved when the Teleri came not, and yet more when he learned that Elwe was forsaken and knew that he should not see him again, unless it were in the Halls of Mandos..." - Of Eldamar and the Princes of the Eldallie
More or less implies some sort of community? They seem to be able to at least see
each other. I wonder how they recognised each other if they no longer had their bodies?:)