I soloed a hard coal chamber yesterday, even got the optional chest(hardest freaking optional ever), got a total of 50 chest renown... and roughly 300-500 monster dropped renown, from 500ish kills.
I think the problem here, might be how "strict" the rules are, for example, Coal Chamber is level 16, and the only time you get the maximum amount of renown is at 16(or lower?). but once you get to level 17, You have a -4% renown penalty.
I think a decent solution to that would be for example.
15 = full renown
16 = full renown
17 = full renown
18 = -2% chance
19 = -5% chance
20 = -10% chance
(this scales like the exp penalty, so it is overall less severe at slightly above over level, but still just as cruel at the end point)
This would help with "end game" renown, since most end game is level 18(non-raids).
Also, 12 man instances are all scaled to pretty high levels, and get people of all levels, It might not be overkill to scale them up to 2 possibly 3 levels for full renown(if possible, else you're going to see ALOT of renownless shroud runs).
Also, if the end reward also scales based upon length/difficulty, turbine, you have to remember, the end reward is always at the same level regardless of which difficulty you do.(change this please ?).