What is so good about chaining them? I usually prefer them separate, for different game stages and corresponding civic changes. Here we are spiritual so it does not apply - but GA ages get more powerful later, with more pop etc
Chaining GAs is for accumulating huge temp advantage, which can make you unstoppable. Like if you are tied up with someone on techs and say you both need 30 turns to get to Rifles. But then you launch a GA and you are say 22 turns from Rifles. That gives you 8 turns of window of opportunity to use your superior units and crush your enemy. But 8 turns is not a lot. Producing enough units (even so much advanced as Rifles vs Muskets) takes time, moving them in position takes time and those 8 turns wont be enough to have decisive win. Where if you chain one more GA, the time to Rifles (including some GP born) drops again, so you have like 15 turns window of opportunity to produce rifles and break you enemy's backbone. When he starts losing cities and resources and territory, he will start to even further fall behind, so you might be able to crush him with the unexpected and undone advantage your second GA can give you.
I am not sure about bulbing engineering mostly for Notre Dame - we can get +2 happy from nationhood (and cheap barracks). But +1 road movement - and trebuchets - are nice for war.
ND is great. We dont have so much happy resources that we can ignore easy +2. Barracks and Nationalism are nice, but only temp solution. If we stay at Nationalism, we will lose our insane Bureaucratic capitol. Right now, I can bet Indira is making about half the beakers for our whole empire.
Wrt to nationhood, RB is clearly preparing for war. Perhaps they want to draft only/mostly in their Globe Theatre City? Then it takes time. They are not that far away from maces, they have civil service. Nationhood also is cheap, gives +2 happy from barracks and +25% EP (only making sense if you run EP slider).
I thinkk Bowsling and others may be right that part of the reason they switch show early is that they want to proceed with their plans - Taj or not.
I was also never sure what is best strategy when you have to go through revolt - do it as early as possible or in the last moment? I understand that for precise answer one would have to compute Return on Investment and such, but was is a rule of thumb at different stages of the game?
Yes, RB is not that away from maces. But I am not sure against whom they are preparing for war. My prognosis that Germans will be their first victim met the unbelief lately because of the Marble-Happy deal with RB. But we know RB have NAPs with all of their other neighbors for the next 25 turns at least (this needs checking and updating to be precise). So, who RB are going to war to? Is it possible scenario that RB went in to nationalism as a diplomatic lever to try and force some of their "troublesome" neighbors in to accepting longer NAP with them? Something like: "We can draft like mad and we are prepared and determined to do so, so if you are smart, you will take our NAP and spare yourself the unpleasant experience of fighting with us".
Or maybe RB are afraid that Poly and CP could take advantage of the big, but very very weak Germans and having no clauses with them to not support their enemies they can use their standing army to keep the status quo?
I think going in to Nationalism is logical, but more about economical move. Their empire is vast (well, not 50+ cities, but with Emperor difficulty and toroidal) and their maintenance for their many cities must be some serious money. With their capitol being underdeveloped (no market, no forge, no academy, not even a single monastery), Nationalism can easily outperform Bureaucratic capitol.
"As for why they switch now?" I think there may be some reason in them closing the limit for 1-turn-anarchy-switch. maybe if they wait a bit more, they will need 2 turns in anarchy for the switch and they prefer to do it now.
faces for Rax... but clearly they dont choose it for this so what is it??
points in all cities... This is why they picked it for sure. Remember that they have TGW. They are preparing for the upcoming War by getting their 
