The biggest problem in maintaining the GDR in some form was that it completely lacked legitimacy.
Yes, absolutely. It was clear that West Germany had no intention of ever fully legitimising it, but I think there's a perception nowadays that the 1990 election there was an overwhelming vote for unification when it wasn't really the case. Only the CDU/DSU wanted it, while the others wanted various degrees of caution. From my books, I seem to remember that the SPD in particular were very unhappy about the speed of change.
I think I said this before, but I think the major mistake was in assuming that East Germany could be integrated into the Federal Republic as the individual states, rather than as a East German autonomous republic. I understand why Kohl did things this way, but 30 years on, it seems clear that East Germany did have a separate sense of identity. If it could be done again, I think the most logical thing to do would have been for East Germany to join the Federal Republic as an autonomous republic - for instance, you could have had some aspects of political union while leaving it to the Volkskammer to decide what to do next with large swathes of the East Germany economy. I'd also argue that the specific nature of East German administration (no states, just counties) meant a difficult transition into the Federal Republic.
Of course, the currency reform was also poorly thought out. It seems that the purpose was simply to make it easier to unify, rather than thinking about what was best for the East Germany economy. One argument is that Kohl knew that by killing the Ostmark, it would be nearly impossible to stop political unification. In hindsight, maybe the best solution would have been to evaluate the East German economy properly before pegging the Ostmark to the DM.
That happened in large because those Western managers had a) better experience in dealing with market economies and b) more importantly often had the money
It's true, but I think it was a fundamental mistake to parachute many Western managers into positions where they had no idea about local conditions. The activities of the Treuhand were never really clear, and the "privatise at all costs" mentality was simply alien to many East Germans.
Personally I think the quick reunification also allowed us to purge SED and Stasi-loyalist from important positions
That might well have been a factor. There's no doubt that the SED/Stasi loyalists were deeply rooted in the state, and Western management at least ensured that these companies were free from them. I know there were some serious questions over the actual strength of the Stasi in early 1990, and I seem to remember being told somewhere (the Stasi museum/old headquarters in Berlin, maybe?) that there was a serious fear that the MfS was maintaining an unofficial parallel structure with the intention of seizing power. They were so deeply rooted in East German society that the only way to really purge them was to have 'foreign' management who wouldn't be intimidated by them. So - from that point of view, yes, absolutely, and perhaps it was worth all the mistakes just to get rid of them.
Reminds me of what the guide told me there - that the biggest mistake people made was to assume that the Stasi were brutes. He told me that they'd found evidence of Stasi men being planted in companies for years, often without the knowledge of the SED management. Apparently they were very proficient at the long game, and they had no problem with someone working for 10-20 years in one place if he proved to be effective in the eyes of the MfS. A state within a state, indeed.