There's no way to know that a miner's hash corresponds to a block template that they show you. OCEAN would only know that their block template or at least one of the transactions within it are invalid...
Of course there is! What would be the point to mining otherwise!
The block hash includes the merkle root of the transactions, so it could be verified easily. And it must be checked! Otherwise how does the pool know to what address you are mining the reward to? Would a pool pay a miner that might be sending the block reward + fees to their own address? You don't need to disclose the whole merkle tree to prove that the coinbase transaction is correct. But the mechanism is there.
In the case of BIP-110 activation, I guess the BIP-110 miners wouldn't want to share rewards with miners that, if lucky, would include "invalid" transactions that they would later reject as a valod block. So then you need to check the whole merkle tree to recompute the root and validate the shares of all miners.
The block hash includes the merkle root of the transactions, so it could be verified easily. And it must be checked! Otherwise how does the pool know to what address you are mining the reward to? Would a pool pay a miner that might be sending the block reward + fees to their own address? You don't need to disclose the whole merkle tree to prove that the coinbase transaction is correct. But the mechanism is there.
In the case of BIP-110 activation, I guess the BIP-110 miners wouldn't want to share rewards with miners that, if lucky, would include "invalid" transactions that they would later reject as a valod block. So then you need to check the whole merkle tree to recompute the root and validate the shares of all miners.