So instead of nerfing the item even further because of the niche cases where it's overpowered, why not simply limit the cases where it actually is overpowered? (Tanky casters you stack mana.)
The following solution to the proc regardless of Muramana's stats:
-Muramana's overcharge effect while toggled on now expends mana and saves the damage into a Charge. All charges are expended upon making an auto-attack against an enemy champion (an auto-attack produces a charge which will be expended by the next auto-attack).
-Manamune can stack 2-3 charges.
-Muramana's damage per charge is limited by 125% of your bonus AD for melee and 100% of bonus AD for Ranged characters.
The following means that:
-You have to balance AD with mana in order to get the most out of this item. Getting Muramana and stacking Frozen Heart/Iceborn Gauntlet and other mana items for a tanky mana bruiser build will not work.
-You have to auto-attack to use the charges, but each spell generates a charge rather than only proccing on single target. This means that in any case where an AD caster has multiple AoE abilities, they won't be penalized. Since AD casters should already be auto-attacking a few times, this will be an overall increase to damage. (Also balances out Urgot with this item as he will not be able to just Acid Hunter an AD carry to death. He'll have to actually auto-attack, so a carry who's running away isn't going to die every time.)
This also allows Manamune (not Muramana) to be unnerfed.