Satoshi used SHA-256 because it was the most widely known algorithm because it was and still is the most used one. At some point you have to take into consideration what's safer: Something that isn't used by X government because it's newer, or something that is used by some government but it's widely known to never have been cracked? I think satoshi did the right thing, rather than gambling with more exotic options, he took the conservative approach.
If there was a backdoor in SHA-256 we would have known by now I think. There's too much stuff out there that uses SHA-256 with big prizes for those that crack it besides Bitcoin, it has never happened, probably never will or at least we will not see it on our lifetimes.
There could be many reasons why Satoshi chose SHA-256 in the first place. One of the main reasons could've been mostly for security, as such cryptographic algorithm is proven to be extremely safe and hard to crack. It seems odd that Satoshi used the same algorithm invented by the NSA, since Satoshi wanted Bitcoin to be extremely independent from middleman or third parties. But, I believe that it could've been for a very good reason.
If Satoshi was a government agent or not, that doesn't matter since Bitcoin's source code is available to the world. In case he/she/they were to crash Bitcoin's price by massively selling the dormant Bitcoin across the market, there's always plan B. The open source and decentralized design of Bitcoin will allow it to flourish across many areas of the world. Other cryptocurrencies can take the same source code, and adapt it to their own needs. Hence, it's not about crashing the price, but rather the technology (Blockchain) that really matters. Just my opinion