Add tests the connection before saving: if the test shows that MySQL rejects the user or password, nothing is saved and the window says why; if the test cannot reach a conclusion, SyncNow asks whether to save anyway.
Test connection checks the credentials you typed and says how long it took, for example Connected · 132 ms, and for a saved connection it also checks the one the agents actually use. Edit changes the host, user or password, while the name stays fixed. Remove database, in the ... menu of its row, asks for confirmation; then agents can no longer use the database, its credential is revoked in the vault, and no data in the database is deleted.
Each change is saved as soon as you confirm it, and the password goes to the SyncNow vault, never to agents. Adding, changing or removing a database is for the owner and the managers. If the database cannot be reached from SyncNow's cloud, SyncNow sets up a small gateway on your own site over FTP, so the project needs working FTP credentials.