1. If you're using a permissioned blockchain, then the "mining" is not about solving a complex math problem, but just about those permitted entities signing off on the blocks they create, and a consensus scheme to ensure that the chain of blocks represents consensus between those entities. Please see the section 'Mining in MultiChain' on page 7 of the white paper to understand the MultiChain consensus algorithm:
2. If the blocks in your permissioned blockchain are generated by multiple nodes, which are under meaningfully different control, and there is an appropriate consensus algorithm – then yes, it is safer than a regular database because no individual node (or small group of nodes) is able to retroactively modify or delete past transactions. That is indeed the point of using a blockchain instead of a regular database – to decentralize control and therefore make the transaction history much harder to corrupt.