A coinbase transaction it not spendable until it is 100 blocks deep in the blockchain. Since a block is produced every 10 minutes, it will typically take 1,000 minutes (just under 17 hours) before a miner can spend the reward.
1). Does the miner get rewarded once 51% nodes accept the block? If so, how is that acceptance information propagated to his/her own node - is it done as and when each node in the network accepts it?
It's 51% of mining power, not nodes. Once 51% of mining power is mining on top of the block, it will continue to get longer at a faster rate than any other blockchain, ensuring it is eventually 100 blocks deep. (This is not really 100% guaranteed, but it works this way the vast majority of the time.)
2). Or does the miner get rewarded the moment he/she creates a block and broadcasts to peers? If so, how is the reward reverted if the block is found to be invalid?
The reward is reverted if the block is found to be invalid because the reward was in that block. So any node that doesn't consider that block valid won't consider a transaction to spend the reward valid. In order for a transaction that spends the reward to be considered valid, the reward itself must be considered valid and the block in which the reward was rewarded must be 100 blocks (or more) deep.
