I think the brief reference to the length of a regah on daf zayin amud alef, as one 58th thousand and 888th of an hour is something important. Can it be that Hash-m's anger is so brief, incomprehensibly so, and yet he should keep us in golus so long? Or can we say that indeed there is no more anger of Hash-m with regard to golus, only rachamim? And in fact the real anger of Hash-m is totally unknowable to any Jew!! Thus since the gemara indicates that if there were really za'am of Hash-m there wouldn't be any Jews left, this must tell us that EVEN during the Holocaust that regah if zaam did not take place??
(a) You are correct that it is Hash-m's Rachamim is in force during our Galus. Furthermore, it was not even His anger that started the Galus. On the contrary -- it was our sins, and as a result of His not getting angry and His having mercy on us, we were spared total destruction and were sent into Galus instead. The fractional moment of Hash-m's anger is cause for destruction (G-d forbid), and not Galus.
(b) With regard to what was going on in Hash-m's mind, as it were, during the annihilation of European Jewry, we are unable to delve into the concealed plans of Hash-m.