I tried to be sure and check out before answering:
https://bitcoin.stackexchange.com/questions/91659/are-btc-private-keys-evenly-distributed-in-256bit-space
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-I think Bitcoin keyspace refers to the possible no. of blocks that could ever be generated till the end of time. (If u know some cryptography before reading about cryptocurrency & the idea of Bitcoin u'll have the impression that mining is similar like performing a brute force attack to break a plain/cipher pair)
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-If Q2:
According to the way a certain block is arranged (what and how many transactions and how they r ordered) u get like a different cipher/plain pair to solve (I mean a different arrangement of bits to search for a valid Nonce)
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I answered because there was a time at the beginning of my reading or u may saying exploring this topic I had the idea that old  failed trials could be saved & used in mining next blocks, now I know this is not quite accurate because each block is constrained by a different transaction & header parts that u have to find a Nonce value that fits them.