I was searching online and a guy said it took him about 50 hours to get to the final boss, and he did pretty much all the sidequests and found all the treasures. I'm assuming that didn't include getting include getting all the endings. As someone who's always concerned with game length I'm pretty happy.
What? The Final Boss can only be reached in Ending D. If he did all sidequests and found all treasures, he's essentially played the game from top to bottom and has it 100%.
Events will change every time you unlock an ending. So you do Ending A and the game puts you back at a certain mission where you continue playing the game then events will have changed to fit the Ending B's storyline. Rinse, repeat for C. Each ending will naturally happen in order.
It's not like other games where certain choices will lead you to a certain ending. You just WILL get Ending A first, Ending B second, and Ending C third.
Aside from Ending D which you have to buy and upgrade all weapons, there is no possible way of NOT staying on track.
Wehrmacht told you this before.