Let's not forget that Trunks is a half Saiyan taught by a Human, they're no Time Lords, I'd take any explanation of temporal mechanics from them with a grain of salt.
Also, just watched Bojack Unbound and it was mediocre, suffering from all the genericness of Super Android 13 with the added bonus of Gohan, who personally I've never much liked, especially when he seems to always need his father's help to accomplish anything.
Apart from the fact the villain took a while to show up this was very typical DBZ movie fodder, evil minions shown to be powerful then roflowned, same with the main "villain".
I'd say those kinds of finishes are the worst thing with many of these films, there are no hard fought fights, just one side (villain usually) smashing the hero and then on a dime (super special power-up ect) the whole thing ends with a punch from the hero. Now I know that ultimately a lot of DBZ and Anime fights in general boil down to this, but usually there is a lot more build up, but in these films there rarely is, due to their length, amongst other things.
Also, Bojack, unlike even Super 13 and Cooler didn't have some impressive transform moment... he just kind of bulked out when fighting Vegeta.