That last point you make seems very convincing to me.
There’s another side as well. To an extent even greater than the Spanish and French parties, though similar to them, it was large enough to generate its own critical mass, something I don’t think is true of the CPGB, or other smaller orthodox CPs which arguably were never big enough (even if they were by contemporary standards of furhter left parties in democracies pretty damn big) to have the sort of tensions and necessity to combine many different interests that made them both cohesive but also attractive to a wider constituency beyond them. And unfortunately the smaller they were the more the adherence to orthodoxy and the inability to develop.
That’s an interesting point about Trotsky. Funnily in the the WP back in the day I knew a couple of people who were more open to him than might be expected given the broader culture of the party.