...pray that german ticket officers understand my situation and allow me to travel anyway?
Well, it is difficult to believe that DB officials will allow you to use other trains, unless there will be a stamp (and maybe some additional piece of paper) from MAV certifying that your previous train arrived late, thus passenger missed needed connection because of the delay caused by partner railway company. But the problem here is that Hungarians might not give it to you even if the train from Belgrade is late, because a) it's not one through ticket (with specified train on some or all the journey legs); b) change in Budapest isn't so called "guaranteed", so it's hard to believe that MAV have any obligation to make sure you make it, meaning that in eyes of MAV it's totally your and only your choice to allow such short change time in Budapest. At least if we were in such situation, then it might be a nice idea to email MAV and ask whether they will be able to help passenger (with two separate tickets) in case train from Belagrade is late. Meanwhile you can just time to time check MAV website site and see how often "Avala" does get late to Budapest, so you at least get some idea on how plausible train change scenario is
Also a small info that maybe be useful in case of short delay (like 10-15 min.): you can win some time if you get off train at Ferencvaros station and then take local bus #103 (run often during the day) to Kelenfold station, that way you can still catch your RJ there
Author