Importing a key never allowed to restore message history from the mailbox. Inbox is treated as a feed and not as a storage. It is possible with some hacks to cause processing of old messages again, but is not guaranteed to have the same result because SecureJoin messages are unconditionally deleted after processing, there is no order between DeltaChat folder and INBOX etc.:
Generally common advice for backups is to test recovery when you take them. Otherwise you might be surprised later that you cannot recover the backups or recovery does not result in your devices being in expected state.
If you manually downgraded to 1.x while keeping the database that was already upgraded to 2.x, it is not going to work as expected. When 2.x upgrade is run, there is a database migration that splits existing contacts into address-contacts and key-contacts. After migration you might have multiple contacts with the same address, some of them having a key fingerprint and some of them not. 1.x is not aware of the “fingerprint” property of contacts and will select one of these split contacts arbitrarily. And if you later import a backup into 2.x or upgrade your setup to 2.x, database migration will not run again and all the changes to encryption state done by 1.x will not be used by 2.x because it stores encryption state differently.
This is why in desktop announcement says that you should not downgrade:
On Android and iOS downgrade is generally prevented by the operating system.
This is not because we want to prevent you from downgrading or force everyone to upgrade to 2.x. As Delta Chat is a primarily a private messenger, someone or some group deciding to use an old version cannot cause any problems to other groups of users. If you have 1.x backup, feel free to restore it and keep using it. Using 2.x database with 1.x client will likely cause unexpected problems that we cannot debug.