I exclusively use beds crammed in at maximum density (1 tile per bed), dwarves are perfectly happy with this and it should prevent vampire feeding, they only get "without a proper room" if they sleep in a bed which isn't designated as their bedroom, it doesn't matter if their bedroom contains 50 other dwarves' beds. There is ONE problem with cramming in beds at maximum density. Sometimes idle dwarves go back to their room to hang out, and if there are multiple dwarves hanging out they'll socialize. Having proper individual bedrooms, or spacing out the beds, prevents this form of socialization.
AFAIK nobles don't get upset by sharing bedrooms, it's just that the room value tends to be relatively low, or if the room value's not low, due to a shared artifact or something, the noble will get upset about the high room value of the other dwarves.
In my experience overlap doesn't matter at all, all that happens is the value of the overlapping tiles are divided over the rooms sharing the tile. So if two beds share a room, each gets half the room value. So there is no benefit in manually making each bed take half the room without overlap - either way, each gets half the value. In fact I believe some tiles, such as walls, are not shared, but each room gets the full value. Hence making larger rooms which overlap, can actually result in increased room value over smaller non-overlapping rooms. So generally 5x5 bedrooms overlapping, will have equal or higher value than 1x1 rooms.
In some distant past versions of DF the algorithm was different and overlapping rooms would kill the room value. But now the value is just shared so there's no harm in overlapping.