What could possibly cause the shift in currency?
During Kimimaro and Mikuni duel Takedasaki spreads rumors to cause the Japanese Yen to become useless thus is's exchange rate with Midas money shrink, this causes the Midas money to appear in Japan to everyone, even some of the bills are half Midas. This was done to weaken Mikuni in his Deals.
The Bank of Midas closes as Japan's Yen has no worth so it can't continue trading, C passes though Japan as it has no worth and allowing Kimimaro to deal the final punch to win the Deal and reverse the Printing Press, returning all the Midas Money back to the bank and getting Japan and it's people's future back.
Japan's economy is rebuilt as since the Yen is useless it adopts the US Dollar, it's believed that also reality has shifted like always to make it so that everything that transpired (C, the falling value of the Yen, discovery of the Midas Money in the Yen) never occurred, this is different from Singapore as it just went bankrupt because it had no mney left while Japan had money, it just was useless. This occurs while Kimimaro is speaking to Midas himself.
Was anything revealed about the motives of Masakaki and/or the Midas bank?
The Masakaki's motives are the protection of their financial districts which is why the Japanese one gets annoyed when the US is able to push C back to Japan.
When Kimimaro wakes up in reality he notices the changes, include where he and Hanabi are total strangers now. The Japanese Masakaki appears apologizing about Hanabi and presents him with a card from the Bank of Midas telling him that the Financial District will always exist, as long as there are futures left to take and if he wants to use the card he can revive the Japanese Final District by calling for Masakaki.
Midas on the other hand, he seems to just put the financial districts out there, allowing people to use them however they see fit well making sure they knew the price was to gain all this money which can be used for anything
As for your question about some grand mastermind, unless there was some group behind C actually occurring there is none, The Starling Guild know about the first instance of C and the effects the Bank of Midas had on reality and was trying to mitigate the damage done, in a sense they were masterminding mostly every Deal to reduce damage, however when C started Mikuni got to rash and was about to do the exact opposite of what he had started the Starling Guild to do.
As for C itself, it seems to be a financial collapse which can ripple though global markets, it's more something that ca occur in reality normally but it's effects are more devastating because C originates and affects the financial districts which in turn affect reality
