Bonus mission 8 is not actually the bonus mission for mission 8. It's the bonus mission for mission 9 (Colditz). Mission 8 doesn't have a real bonus mission like the others. So don't panic when the game starts with mission 9 instead.
I ended up replaying mission 8 two times, because it wouldn't start the invasion in the end (it was bugged), which is initiated when you collect all intel.
The second time, I had a save just before picking up the last intel for the bonus missions and learned that you do not need to do the invasion at all.
Just go for the primary objectives. So do not bother collecting the intel. Save yourself the hassle. If you do feel like trying, and since I could not get the blowtorch to work on the downed plane, there is actually another way in from behind if you dive down). The remaining intel is in this downed plane.
In regards to this mission, it's also worth mentioning that the the Tank cannot shoot. It works in the PC version, but not the console version. So don't bother. I spent half an hour trying to get it to shoot - it just doesn't work.
Instead, go grab the rocket launcher from the box in upper corner and shoot the closest tanks and use mines (which are also found in some boxes) on the rest.