You could just treat the archers as a flame/spore attack for 2 points. That would give you a pretty powerful 10-point profile when combined with Fear.
The problem is the Wild Charge. Adding the ranged attack loses the Wild Charge (which surely defines a Mumak!)
I think it needs something else. I'm thinking use the Were rules. So the unit starts the game as an archer unit (counting the houda as cover) but when it takes damage it can run amok and is transformed into a Greater Warbeast with the caveat that it will Wild Charge friend or foe alike - going for the nearest unit if it has a choice. It causes fear but is also fearful. So you have a wildly unpredictable unit that could do what you want or could ramapge through half your army at the first whiff of danger. Would be fun(ny?) to play anyway!
I'm not so sure about Wild Charge defining a mumak. The mumakil don't do much charging in The Lord of the RIngs, at least not before they're routed. They actually seem to be quite well controlled by their crews and pretty steadfast:
"But wherever the mûmakil came there the horses would not go, but blenched and swerved away; and the great monsters were unfought, and stood like towers of defence, and the Haradrim rallied about them."
On the other hand, your suggested rule makes sense for when they do go wild (as seen by Sam in Ithilien). But I think it loses the "towers of defence" aspect - unless you define the archers as being in superior cover (like castle walls) from the get-go.
Here's a thought: what about using the Greater Warbeast + Flame/Spore Attack (archers) + Fear, but adding a rule that when mumakil are have no room to retreat, the blocking units (whether friend or foe) take the damage that the mumak would normally take (based on the single-die Courage roll). So, a routing mumak could be advantageous, in that it might blunder into enemy forces, but it could equally (and more likely) be damaging to its own side.
That would also have the effect of making enemy cavalry even more discinclined to engage! Cavalry units can be pretty useful at "outflanking" enemies in DR/LR, by getting in position behind enemy units, who will then suffer from no room to retreat if pushed back. But you wouldn't want to do that with a mumak ...