Then you don't need blockchain. Once you have trust among the participants, you don't need blockchain.

as i said - I'm more interested in a immutability of the whole chain, without any backdoors to modify old data because - speaking form experience - that leads to way more problems when dealing with local governments.

And the point myself and others are making is that you can get that without using blockchain. Blockchain is a specific technology that requires proof-of-work among distrusted actors for modifying a cryptographically secure distributed ledger. It sounds like you just want a cryptographically secure distributed ledger.