MS SQL "migration" to older version - problem with Attach
Question 1 - NO
You cannot restore or attach a database from a newer version of SQL to an older version.  The only way to go backwards is to use a bacpac to export the schema and data and then import again.
Question 2 - Compatibility Level
This mainly controls whether certain features are enabled on the database (such as JSON), and it also controls which cardinality engine is used for estimating plans and their various costs.
Question 3 - How to go backwards
I haven't used the tools you referenced so I don't know about them.  But I have used bacpac (included with SQL Server) to move databases around before.  It works better with smaller databases.
You can also use SSIS to migrate the data although that may be a lot of work compared to just using a bacpac.
https://www.sqlshack.com/importing-a-bacpac-file-for-a-sql-database-using-ssms/