A single database restore is not the operation you need if your goal is the merge data. You are looking for a data level solution not a database level solution.
SQL tooling can help you with this task, but it is not required (it depends on knowledge and skill level). The 2 data compare tools I have used Red Gate SQL Data Compare and Apex SQL Data Diff have ...
When upgrading any software it's usually a good idea to check the manual first, and read something like this:
To upgrade an existing MongoDB deployment to 3.6, you must be running
a 3.4-series release.
To upgrade from a version earlier than the 3.4-series, you must
successively upgrade major releases until you have upgraded to
3.4-series. For example, if ...
You cannot jump directly to version 4.2... You must do upgrade step by step, increasing version from 3.6 to 4.0 first and then you can upgrade it to 4.2
At every step, you must upgrade featureCompabilityVersion.
There's a third-party free tool (albeit from company that contributes to Postgres) named pg_probackup that allows to restore specific databases from the cluster. As I understand it, the backup operation still works on cluster level. I should check that out
This no longer appears to be an issue. I have restored a database to the same server with a different name. It shares the same file_guid however and both databases are online without issue. You can use the following query to quickly see the files and locations of the databases on your rds server.
SELECT DB_NAME(database_id) AS DatabaseName, name AS ...