Reeen
For the rewards mechanisms I'd propose HoL model. 20 distributed challenge levels (so roughly every 5 floors) and the distribution is exactly proportional to HoL (assuming it to be in line with whatever math used by Gravity) circulation of life reward
1st= 15 = x1 base rewards
2nd= 23 = x1.53 base rewards
3rd= 32 = x2.13 base rewards
4rd= 42 = x2.8 base rewards
5th= 52 = x3.46 base rewards
Etc
So for example in the case 1st checkpoint (at floor 5) rewards 1 special instance coin, 5th checkpoint (at floor 25) rewards 3.46 special instance coin. Also probably will need to introduce special instance coin fragments item that can be combined onto whole instance coin.
At max checkpoint (346, of which means x23 base reward) you will get 23 instance coins but I don't think it will be viable for a couple years minimum. Realistically people will mostly able to reach midway at around 10th checkpoint (118, of which means x7.8 base rewards).
Also the damage reduction increase could also follow these checkpoints. Also that more checkpoints are better at more precisely rewards people of where is their standing in the bracket of power/relevant skills as a player.
Also since it is every 5 levels, I'd propose a minimum of one range, minimum of one melee, and minimum of one magic floor among those 5. This is in accordance with the suggestion to reduce mobs for these type of floors (damage restrictions floors). Otherwise unnecessary if the mob reduction change is not implemented