This recent update brings an overhaul to the Barrows minigame. The entire thing has been overhauled to be correct to RS2 / OSRS - the game during that era. For general answers about rewards see: http://2007.runescape.wikia.com/wiki/Barrows#Kill_count_and_loot
And for a period accurate guide see:
So getting 14kc doesnt help the chance to get a piece now? I should just
get 6 kc if I want a chance of getting every bro's piece?
No. The formula is entirely dependent on how many brothers are dead. Each brother you kill rolls for barrows pieces once, so 6 rolls times the chance per brother is the maximum chance for -a- piece of barrows.
Aren't there supposed to be ladders in the NW and SE rooms?
No. The ladder spawns in the room you go through the empty crypt into. You either teleport away after opening the chest or re-trace your steps back to the original room you spawned in and climb out of the tunnels.
Will brothers still randomly spawn when trying to open the chest?
Yes and no. It's not entirely random if you haven't killed all the brothers before searching the chest. If you run to the chest with 1 brother still alive, that will be the one who appears at the chest. Likewise, if you search the chest with more than 1 brother alive, a random one will spawn. This mechanic did not change from the old Barrows.
Any benefit to raising kc, like unlocking bloods?
Yes, raising killcount to 8 unlocks bloods and bolt racks per realscape. Raising kc to 14 unlocks dmeds and crystal key halves. Higher killcount ensures better rune / rack / random rewards.