If Britain stood Catholic, Netherlands would have not existed in the first place
I am not so certain about that. Yes there would be a large number of butterflies in the war, but assuming England stays out of the Dutch revolt, it is still possible for the Dutch to win. The English did not give much support to the Dutch (and what it gave was often contraproductive) and I agree they were a nice distraction at the right time during the Armada; the Spanish war with France was the deciding moment of the Dutch revolt. Although hardly a certainty, an catholic England does not mean that the Netherlands never arises. Especialy considering that out of realpolitical reasons (the Habsburgs are very strong and are a threat to England too) they could still decide to support the Dutch; the catholic French did after all.
and second a Catholic Britain would irreversibly change the outcome of the (definitely less than) 30 Years' War to a decisive Catholic victory, what does this mean?
Assuming they get involved and get involved on the catholic/Habsburg side. Both are possible, but hardly certain. It is pretty likely they decide to sit that one out (like OTL) or even to support the protestants, hoping to weaken the Habsburgs, just like catholic France did.
Simple, the Protestants will be forced to convert back to Catholicism.
You mean killed, I assume.
Anyway, you are right that it is possible there will be no protestantism, or it is a lot weaker than OTL. But it is possible it still manages to exist and even be strong. Maybe as a counterbalance to being surrounded by catholic, (pro)-Habsburg nations France switches religions or maybe a Austria switches religion. Or France manages to beat Spain at some point and manages to conquer the Southern Netherlands (or all of it if it remains Spanish). Or whatever. My point still stands. Europe will be very different, if England remain catholic and before we can reliably speculate about a war of Spanish succession, we do need to know more about it.