Goblin hearts can only be turned in to Richard in Broken Valley. This will close the quest (or changes the colour, anyway), but they next heart collected (in Broken Valley) will open it again. If the quest is opened when Richard is no longer available, it can not be completed.
The killer bunny is level 22, so the maximum experience bonus you can get is at level 19 or below, with lower levels giving potentially lesser loot.
With my ranger character in DKS, I headed to the fjords at level 13; clearing just past the teleporter brought me to 14, I hit 15 part way down, and the first 2 opponents on the opposite beach when I got to the bottom got me to 16.
After that I went back to Broken Valley to go through the Temple of Doom (bandit camp) and Maxos Temple, then (on level 17) returned to the fjords, and got to level 18 after swimming over to the beach near the anti=dragon defences. Clearing the path past the depleted ore mine brought me to level 19, at which point went to go kill the last rabbit I needed to trigger the killer bunny.
The first level of the mine brought me up to level 20, and the remainder almost got me to level 21. I leveled after the first 2 opponents in the waterfall/Patriarch cave, and got half way to level 22 finishing off the accessible areas of the cave. Since I could not reach level 23 by turning in quests, I reloaded the save outside the waterfall, and (after turning in all quests to check experience in the level 21 save) figured out which quests to turn in to just reach level 22 after re-clearing the caves, to get the most experience possible with the first mindread on Sentinel Island.
In the original release of D2:ED you could get into the mining tower early. In DKS and D2:DC, the tower is locked until after you become a dragon knight.
