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In general, in order for a thread to interact with any resource external to itself (such as disk I/O, other processes, other threads, etc.), it does so through the use of system calls. There are many system calls, many which could be dangerous in the right context, and many which a thread is not going to use. Syscall sandboxing thus restricts the system ...
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No, when you buy Bitcoin from an exchange, it is not an on-chain transaction, the exchange simply updates your account balances in their database.
It only becomes an on-chain transaction when you either deposit or withdraw Bitcoin to or from a Bitcoin address, from your exchange account.
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The security of half a key would have to be more carefully considered. Suppose the key is n-bits long, and you know n/2. This paper suggests that generically, Pollard's kangaroo algorithm would mean you have n/4-bit security (i.e. an attack running in expected ~2^64 operations). However this doesn't mean that's the best attack possible, and it may be better ...
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