Hello,
Some of the answers are in threads you linked, but I just wanted to emphasize - you can really do anything you want in regards to integrating other application with EPESI. As long as you can access the interface through some kind of API on php level, it's good to go. Going to the total extreme, you can consider using exec() method, but make sure you safeguard it best you can.
As for utilizing EPESI features - pulling out records from another database is one thing and it can be done with QFfield/display methods you've seen in the other thread. If you want to be able to affect the database, but you prefer to rely on EPESI CRUD engine, it also supports trigger-like calls, allowing you to update external data every time a record is saved. More information available here:
http://www.epesi.org/index.php?title=Ut ... riggers.29
Best luck on your project! 🙂
Arek