Nice scenario! So lets say the next few years Rome and maybe Carthage, attack Megale Hellas League. What would Massaliot League do?
First I reckon they'd cut off all trade with the Romans, and turn back any ships trying to trade with the Romans where they can, and flat out raiding Roman shipping. The League has naval superiority, perhaps over both the Romans and the Carthaginians, but at least each in turn.
Massalia has shown they are capable of invading Rome by sea previously, and can probably do so again to take control of Liguria and Etruria. This would at least create a second front for the Romans to deal with.
Carthage is the real prize for Massalia in such a war however - if the troops can be spared, an invasion fleet to capture the Carthaginian coastal towns would cement Massalian dominance of trade in the W.Med forever - and advantageously Carthage has less money than ever before, which could lead to the Massalians being able to turn the Mercenary armies of Carthage against them - that coup would turn the war in Africa into a rout - that would then enable Massalia to send mercenaries to help in Italy.
The Megale Hellas Leagues survival will rely on the success of the Northern Campaign in distracting the Romans, and their own efforts to slow them in the south. If they can hold out long enough for Carthage to capitulate, then have those forces turn the tide.
This doesn't mean that Massalia is fine however - even in this circumstance, to buy out the mercs, and maintain their armies means stressing every resource an already extended Massalia has - I fully expect riots throughout Iberia - perhaps even an attempt by Hannibal and the Romans to sneak Hannibal across to Iberia to cause an insurrection - this third front is the real danger - if the Massalian League can't muster the resources, be it financial or military, to prevent this uprising, the Megale Hellas League is at huge risk, as that reinforcement army of Mercs in Africa, will be going to Iberia.
However, this all changes if somehow the Ptolemaic Empire is involved - getting them alongside would be great - difficult because of Megale Hellas though. A large relief force sent by the Ptolemaic Empire would be able to not only hold the Romans back, but perhaps push them back entirely. It may come at the cost of giving Sicily to the Ptolemaic Empire to ensure their assistance.
I can't see the Megale Hellas League being a fully coherent entity after this war - either propped up by Ptolemaic or Massalian troops. I don't believe that the Empire is interested in Italia - they have their own issues out east - which could mean that the Megale Hellas League are essentially annexed by Massalia. Alternatively they are asked to choose a leader, perhaps the King of Epirus.
Assuming Massalia, this leaves a colossal, largely overstretched League - that will be able to do little else at the moment beyond stabilize, and maintain a strong trade relationship with the Hellenic Empires of the East - and even encourage their migration to the West, so that there can be good Greek-speakers (which short of a Massaliote is the next best thing) throughout the League. The mercenary armies of Carthage remain for a long time in the service of the League, expensive peacekeepers that have little to no loyalty to the League, or its people - but useful to stomp rebellions.