I love how this timeline really leads to such in-depth analysis. Your arguments are very compelling,
@Shevek23! I think you are right, although I must caution against a sort of 'determinism'. What Rome was like in OTL is not by definition what it is like in TTL. What really matters is how strong they are at the moment. What will a war with Massalia do to them? At this moment, I don't see them winning. I don't see them going south (to Megale Hellas) first, either. They will want Cisalpine Gaul, because if Massalia grabs it first, that's even more of a threat to Rome than Rome having it would be to Massalia. My initial thinking was that Massalia can fortify the mountain range and thus be pretty safe from overland attack, even if Roman territory is nearby. (But it seems I was wrong to brush off such corncerns so quickly.)
It remains my thinking that a war with Massalia is not really in Rome's interest. Nor in Massalia's! Say... would it be an option for both Rome and Massalia to guarantee the independence of the Cisalpine Gauls? To turn the area into a neutral buffer state they can both trade with, but which both solemnly vow not to occupy? It would be tricky to arrange, but it would avoid a very costly war that neither side can afford. And its exactly what the local Gaulish population want, of course.
In such a case, I still think the rest of my scenario is plausible. We must not underestimate the value of the east. Rome could gain a lot there, and even if they headed west first in OTL, I again urge against determinism. If there's something to gain in the east, they might just take that opportunity. It's a cost-benefit thing. Sure, they'd gain a lot if they could subdue Massalia and capture its riches.
If. It's a huge rist, they might not want to take.
Regarding the Ptolemaic Empire being set against the Romans adventuring in the east: initially, they'd be able to gain from allying with Rome and dismantling Epirus, Sparta, Rhodes, Megale Hellas etc. In the long term, they might become a target themselves, but it would hardly be unusual for a ruler to accept such a distant future risk in exchange for vast benefits in the immediate future.
Final note: messing with Megale Hellas might indeed incur the wrath of Massalia. If I were Roman, that would be a prime reason to secure the cooperation of Ptolemaic Egypt (also vaguely allied to Massalia) to make League neutrality more likely. Also... I'd wait until the inevitable moment when massalia gets into a war with the Barcids (because both are making inroads in Iberia, they'll soon come to a clash). They'd be distracted. Perfect moment to scoop up Megale Hellas.
All my thinking here goes towards the idea that a Roman-Massalian war would be extremely costly at best, and devestating at worst. I really think both sides would know that, and try to avoid that war. At least for the moment.