Well, firstly, there is pretty much no opportunity to make the audience think the Green Elves have a chance without expecting the audience to be great fools, and suggesting that Thingol and Mablung are even bigger ones.
Secondly, there are a lot of things I would change with this project. My question is: what do you suggest to do now?
Here's how Tolkien describes the battle:
Now the Orcs that multiplied in the darkness of the earth grew strong and fell, and their dark lord filled them with a lust of ruin and death; and they issued from Angband's gates under the clouds that Morgoth sent forth, and passed silently into the highlands of the north. Thence on a sudden a great army came into Beleriand and assialed King Thingol. Now in his wide realm many Elves wandered free in the wild, or dwelt at peace in small kindreds far sundered; and only about Menegroth in the midst of the land, and along the Falas in the country of the mariners, were there numerous peoples. But the Orcs came down upon either side of Menegroth, and from camps in the east between Celon and Geilion, and west in the plains between Sirion and Narog, they plundered far and wide; and Thingol was cut off from Cirdan at Eglarest. Therefore he called upon Denethor; and the Elves came in force from Region beyond Aros and from Ossiriand, and fought the first battle in the Wars of Beleriand. And the eastern host of the Orcs was taken between the armies of the Eldar, north of the Andram and midway between Aros and Gelion, and there they were utterly defeated, and those that fled north from the great slaughter were waylaid by the axes of the Naugrim that issued from Mount Dolmed: few indeed returned to Angband.
But the victory of the Elves was dear-bought. For those of Ossiriand were light-armed, and no match for the Orcs, who were shod with iron and iron-shielded and bore great spears with broad blades: and Denethor was cut off and surrounded upon the hill of Amon Ereb. There he fell and all his nearest kin about him, before the host of Thingol could come to his aid. Bitterly though his fall was avenged, when Thingol came upon the rear of the Orcs and slew them in heaps, his people lamented him ever after and took no king again. After the battle some returned to Ossiriand, and their tidings filled the remnant of their people with great fear, so that thereafter they came never forth in open war, but kept themselves by waryness and secrecy; and they were called the Laiquendi, the Green-elves, because of their raiment of the colour of leaves. But many went north and entered the guarded realm of Thingol, and were merged with his people.
We should show all of this. Here's how I would do it.
1. All the things we discussed about how the Green-elves' battle are good. I would keep them all, including Denethor's early departure and the Saeros plot.
2. The battle story should start with the invasion of the orcs into Western Beleriand. We have this happening in an earlier episode, showing the action at Eglarest. This works. Someone comes to Thingol to raise the alarm. Refugees from the "Elves [who] wandered free in the wild, or dwelt at peace in small kindreds far sundered" race into Doriath. We can show the orcs destroying their villages and chasing them (cf. Jacksons Two Towers Rohirrim refugees). Thingol then sends Mablung to Ossiriand to raise the alarm to the Green Elves. This would be a good cliffhanger.
3. Then the orcs attack Doriath.
Thence on a sudden a great army came into Beleriand and assialed King Thingol. The action starts. Thingol isn't quite ready, but his army is superior to the orcs and withstands the first attack. He doesn't know the full extent of the orc army. The orcs army continues to grow and Thingol is beset
upon either side of Menegroth.
4. Meanwhile in Ossiriand. Mablung arrives and begins to assist Denethor to prepare for battle. The orcs show up soon thereafter. We follow the plot discussed previously. Denethor dies. Mablung assumes leadership as field general. The Green elves begin their retreat to Amon Ereb. We can show a rearguard skirmish here with lots of woodland trickery.
5. Meanwhile in Doriath we have a sweet battle scene. Thingol's well armed elves fight against the well armed orcs. This should be lots of fun to watch. Lots of cool battle stuff. Siege engines. Hand to hand combat. Shiny swords and spears! Superior elf discipline defeating disarrayed orc barbarians (e.g. the elf front line in Jackson's battle of the Last Alliance which looked really cool before Sauron showed up). However, the orc army is still huge and Thingol hasn't been able to break out.
6. Meanwhile in Ossiriand. Mablung and the green elves arrive at Amon Ereb. The Green-elf rearguard is finally defeated. Amon Ereb is on a hill but the fortifications are weak, and the orcs slowly penetrate the defenses. I forgot what we decided to do with Mablung and Saeros, but much of their character arcs can happen here.
7. Meanwhile in Doriath, we conclude the battle. This is a long scene with lots of action (remember: Silm Film has a huge budget). The orc army attempts a final assault but the heroic elves stop it, and it finally turns into a rout. We can sacrifice one of Thingol's generals to save Menegroth from the assault. Thingol begins his march to Ossiriand.
8. Meanwhile in Ossiriand the orcs continue to attack Amon Ereb. There are some heroics, but the Green-elves lose. This plays out as previously discussed, and we stay in the Green elf perspective until the end of the battle and the ents' arrival and Thingol's arrival. Thingol's army and the Dwarves finish off the orcs. Sauron is unhappy.
So in the big Battle Episode there are 5 changes in perspective, which shouldn't be too jarring for the viewer (we're not reaching Return of the Jedi level confusion). We get lots of fighting. We get some ebb and flow. We get the uncertainty of both Thingol and the Green elves about what is happening on either front.