You are correct that for single currency data, only the Chart of Accounts and Batches are available for importing - there is no direct import available to the Transaction History. You are also correct that if the history from the other database was created as an Excel or CSV format with a proper layout, it could be imported as a batch and posted. This would really only be for current year. You could also import prior year numbers using the import COA function.

If you have a multiple-currency company, then importing source currency information for your accounts is available, though likely this won't help your particular situation.

As for the import batch layout itself, there is no one specific way but rather you may customize what the layout is. You can select which fields to import and in what order - not stuck with the one way defined in a program. If a field is exported by the other program and has no respective field in Adagio Ledger, you can set the import to skip those fields, rather than having to edit the raw import file. The best thing to do is take a look at the available fields on the second screen of the import batches function.

If you are importing an Excel file, use the Import Batch - Detail Only format. If a CSV file is imported, you can use either.