By itself a block chain just stores hash with each block, they could be recomputed after a change. Just like how Git history can be changed.
Unless you're talking about a distributed public no-trust ledger, yes that can be solved with block chain as it's what Bitcoin is. But that's not a problem they're having.
Sure, but that’s only needed in a purely distributed environment. There is absolutely no need for a blockchain in a centralised private database you have full control over.
Reading the page, that's exactly it; journal entries are an append-only log.