Hello Richard,
OK - hopefully this helps

1) There's no specific significance on this date (other than it sets up a default fiscal calendar). This calendar is overwritten when you select data to retrieve (based on the year you select).
2) Adagio FX databases are made up of a collection of files (unlike QB which stores all its data in a single file). Generally, I would recommend creating an FX subdirectory under the directory that stores your clients QB database.
3) When you want to retrieve data, specify the fiscal year you want to retrieve (the year in which it starts). Adagio FX will retrieve that fiscal year, and one year prior.
4) At the moment, Adagio FX only stores two years of data directly. We expect this restriction to be lifted some time during this year. A simply workaround, if you are not using classes in your database, is to create a "department" for each year and retrieve the data multiple times from the same QB dataset appending (this is an option on the retrieve dialog) each year into a different "department". You can then easily compare each "department" (which is actually a year) on the same statement.
5)If you have specified account numbers with your g/l accounts in QuickBooks, then tick the option "User defined accounts" on the retrieve dialog. FX will then use your account numbers on each retrieve and warn you if it encounters an account missing a number.
6)When you retrieve data for a second time, Adagio FX does overwrite the old data (but not the financial statements you've defined). If you used the trick of importing the multiple years into different "departments", then you'd select an option on the retrieve dialog to delete the current year's "department" before doing the retrieve.
We do understand that the performance of the retrieve leaves something to be desired. Unfortunately, we are constrained by the performance of the Quickbooks SDK, and we have confirmed with them that there is little we can do further to improve the retrieve performance. We do know that Intuit is aware of the issue.
Thanks for giving Adagio FX a try, and feel free to keep posting questions on this forum. There's a learning curve at the beginning, but it's worth it at the end!