driving Mostar to Zagreb

As a starting point, I suggest seeing what ViaMichelin.com offers as a driving-time estimate. That will not include any breaks or border delays, and folks seem to think VM tends to be optimistic, but it will at least give you a rock-bottom minimum.

I would use Google Maps and just pad the estimate by an hour or two to be safe. Know the driving estimate to Zagreb from your last stop. For example, Google Maps (checking now) shows an estimated drive time of 3:32 from Jajce to Zagreb train station. If I had to catch the train at 21:20, I'd probably leave Jajce by 4pm (and with zero delays, assuming Google is correct, you'd arrive at the station by about 7:30PM. That assumes you drop the car right at the station; if not, then maybe leave at 3:30.

I didn't drive directly from Jajce to Zagreb - I detoured to Plitvice Lakes National Park in Croatia first for a night. But I drove the length of Bosnia. The roads were all quite good - much better than expected, given the post-war reputation of Bosnia having terrible roads. I crossed the boarder at Bijac, but you'd probably drive via Banja Luka and cross up there. Border crossing was not long for me, maybe 10 minutes tops (this was 2015).

I did drive partly toward Banja Luka from Jajce - because I had heard the scenery was really pretty, and it was.

My experience is similar to Andrew's. Fine roads throughout B-H and no problems or delays crossing the border. Just be aware that the roads are mostly two-lane higways (not limited-access multi-lane freeways), so don't expect to go much faster than 55, and you will need to slow down as you go through villages, small towns, and cities.

I drove from Plitvice to Sarajevo in 2014, with a stop in Jajce. I don't remember how long it took, but it was quite a bit longer than I anticipated. But the scenery was wonderful!