I never got the "Star League is too expensive" excuse for disbanding The Star League, since you could just move to reduce your commitments now that you've got twice as many votes. But whatever. I think the reason The Jihad started is the same reason the Word wouldn't have gotten far enough to actually assault the Clans. They were several camps of zealots with WMDs wearing a trenchcoat to look like one entity. That's not a recipe for a stable actor, much less one that could plan and execute an Uno Reverse Clan Invasion: War Crimes Edition.
Scenario I: The Star League Plan
If they did throw all that stuff at the Clans in lockstep with the Star League, they'd probably be doing it with minimal help of the LC, CC, C*, and FS. I'm sure the LC would have a vested interest in giving whatever they could do. The CC could be coaxed into contributing something, and obviously ComStar would be pretty reluctant to supporting a WoB operation.
So they'd be cribbing Victor's notes to do Operation Bulldog, mk II, probably against the Falcons and/or Wolves (we can't do all of these operations to help the DC). Under Vlad Ward and Marthe Pryde, they're the two canniest Clans, the ones most likely to be on the lookout for another Operation Bulldog, and the ones likely to do what The Clans wouldn't do for the Smoke Jaguars: cooperate. Meanwhile, the Word would probably be using their war crime-iest tools on their own Operation Scorpion, against pre-Reaving Clan Homeworlds.
I'd imagine a bogged down Star League operation--surrounded by mauled, angry Falcons, Wolves, and (possibly) Bears, on the hook for Blakist atrocities in much the same way the Blakists hooked themselves in The Jihad--holding dicks in hands when homeworld remnants come screaming out of the Periphery with worse horror stories and nothing to lose.
If the Clans come out on top, Great Refusal or no, you'd likely see a coalition of Clans--all of them--driving for Terra and The Blakist Protectorate. Do not pass Jihad, do not collect 200 Stone, go directly to ilClan.
Scenario II: A Very Blakist Operation
Sans the Star League, I'd see fewer operators being involved. Maybe just the DC, FWL, and LC. But without a Star League-level buy-in, I don't see even those guys throwing as much in. I see a plan buoyed with more fanatical optimism, fewer seasoned hands at the till, and less brute force going south sooner and harder, but bringing far fewer people down with it.
If the timidity of the participants is enough, they might--might--delay it until the Wars of Reaving and absolutely shatter the homeworld clans while they're already tearing each other apart. If that happens, there's a chance it would actually provide the kind of external threat necessary to reunify the homeworld clans and 'cleanse' them by giving them the shock of a depraved, spheroid war and reaffirm The Way of the Clans. My understanding of the WoR was that it was a society based on their aggression and superiority being unable to reconcile a resounding, undeniable defeat and having nothing to do but turn on scapegoats (or reconcile flaws in their society, but you can't do that with a Warhawk, so...).
Slim chance in a slim chance in an improbability, the homeworld clans might win, but mostly we're turning the dial on the number of homeworld Clanners getting to the Inner Sphere and whether they're refugees or reinforcements. The "How Mad Are They" dial is pretty much the same (11/10). I still don't see it turning out great for the Word--possibly worse--and it's definitely worse when everyone in the Inner Sphere sees the Blakists' true face.
I don't see how a Blakist Coalition could win without the clans being incredibly stupid (so...60/40). If the Clans are strong enough to drive to Terra, they will. If not, I can see the DC, FWL, and LC offering the depleted Blakists up on a plate to the Clans for peace. Blakists are arrested and tried (probably by ComStar), The Blakist Protectorate is picked apart like the Hegemony before it, and Terra is returned to ComStar (the spiciest of options).
Scenario III: Blakists Are Doin' It for Themselves
If the Word tries to truly solo a invading clan, are they still splitting forces to go after the homeworlds? They'll still face backlash if they start dropping Shadow Divisions, mass drivers, and WMDs willy-nilly in a Clan OZ.
The only success I see for them would be if they shop around for allies and get nothing after years of haggling. They put everything into an Operation Scorpion II in the homeworlds, which arrives during the WoR. The homeworlds are obliterated, with sparse and inaccurate data percolating back to the Inner Sphere. It becomes a mystery on par with the Minnesota Tribe.
But the original question was 'if they hit all the OZs.' They'd get creamed.
If they hit one OZ, I think they'd be successful, depending on the willingness of the Clans to recognize a common threat. If they used all of their resources, they'd lose the trust of much of the Inner Sphere. If that didn't end with a Jihad, they'd probably retreat to the hidden worlds and become a ubiquitous religion with extensive technical knowledge.
Answering the Questions
How many IS civilians would die? A lot. More in scenarios where the Blakists fight on their own and lose. Fewer in scenarios where they're part of a coalition and (somehow) succeed.
There wouldn't be a Devlin Stone or a Republic. Either the Blakist Protectorate would spin off into its own thing or it'd be picked apart (again). One in a hundred chance the Blakists, lose, call the game, and leave a token force--with leader--to hold the Protectorate together. That leader would probably be Blake and he might live or he might die, but I doubt he'd save The Protectorate. But without general war weariness or a thorough ravaging of Terra and surrounding worlds, I don't see the Successor States abandoning those worlds to a power that isn't even a particularly good buffer state.
Without being implicated in Blakist actions which directly affected their neighbors and their citizens, the FWL would probably hang together. There might be some problem rooting out Blakists if their alliance becomes detrimental, but the FWL has had civil wars before and most scenarios I see end up with the Blakists being depleted by the Clans and not being able to be a significant player in IS politics.
Would the Dragon take time to extend in to the Federated Suns? Probably not. In a lot of scenarios where the Wobbies have any allies, the DC is nominally one of them. ****** up the Clans has worked out well for the DC against the Smoke Jaguars. Helping to destroy the Wolves or Falcons doesn't help them as much short term, but if gangpiling clans is going to be a 'thing,' the DC is just waiting for the Ghost Bears' turn to come before they dip on the concept.
