No, none at all-your findings are correct. From black mountains (=Cerna Gora=MOnte Negro)_ to what is now Bosnia has never ever been a raillink. Only remaining is the line to Beograd-someone posted recently that now that summer is near over only the overnite remains-daytime no more (but unconfirmed). This is also the only remaining raillink OVER borders in what once was Yugoslavija (except Hrvatska-Slovenija, both EU)

I'm not sure what you mean. There is a confirmed timetable:

https://www.srbvoz.rs/wp-content/redvoznje/ms_en.pdf

On page 12. The day trains run until 18 (to Bar) or 19 September (to Belgrade).

Hello! The reason why you don't see any trains on the timetable, is because not all train carriers share their timetable with us.
I would advise you to check timetables and updates by the railway companies of each country respectively and book via their website or locally at the train station.

If a connection does not appear on the app, try to add this journey manually: 
To add a journey manually, please do the following:
At timetable section, you may type in some random characters within the "From" field. Because the timetable won't recognize any stations, it will give you the option to add a station manually. Click on this option and you can proceed to enter the journey details manually, and then add it to your pass so as to have the right QR for travelling.