It doesn't need to be a home clan invasion. It could be as simple as the Adders (along with their auxiliaries - the rest of the homeworld clans), pop in en masses to the Terran system, say "hey... we'd like to reach an accommodation.... we'll be your armies, let the Republic become the TH 2.0 and we can form a Star League with the Republic/Hegemony at its head and we'll become the SL Army again. Jobs a good un, no hassle no grief, anyone disagrees we've got a lot of troops to say otherwise".
Reform the SL, reorg all the factions, with Clan Star League Adder as its army, the SL armed forces go skelp some upstarts around, peace in our time, jump a bit in time, as the existing factions have old dying leaders, new leaders have come into the fold ... tensions threaten to break apart the SL....
Reboot the whole danged settings in a believable way. People still have some skin in the game, as their faction wont be *gone* just slightly different.