Assuming you know who everyone is, you can use traditional consensus mechanisms like Byzantine Paxos and get much higher transaction rates than public blockchains can do right now.
Not really, you can have a blockchain managed by a single company. In the most basic definition, a blockchain is simple an authenticated linked sequence of records. It can be public or not, it can be processed centrally, distributed across a single organization, or distributed in a peer-to-peer fashion.
In your example of multiple companies coordinating on a single blockchain, who would be paid to manage it?
If a block isn't finalized, and you build a transaction based on particular conditions in that block, and the finalized block has different conditions, then your transaction just fails.
The same data in a specific block, but not everyone necessarily agrees that the same block is the latest one. That's why you don't have 100% confidence in a single block confirmation.