Maybe I'm wrong, but this is the way I understand the rules :
- We talk here about difficult terrain, not dangerous one (jump infantry may land upon difficult terrain when charging, but takes then a dangerous terrain test that does not change the nature of the terrain : it remains a difficult one).
- Charging through difficult terrain lowers the models to I1 for the fight (except for the models which have grenades). Jumping above it does not.
- If the jumping models land upon difficult terrain, they are affected by it (lowered to I1 for the fight, except for the models which have grenades). Moreover, each model must pass a dangerous terrain test (even if the terrain is just a difficult one).
- "Under cover" is a difficult terrain. So, if the unit charges its target through its cover (or lands upon it), i.e. if the straight line of any model passes through the cover, the whole unit is subject to difficult terrain.
- Charging through dangerous terrain is the same, except that even models which don't jump must pass a dangerous terrain test.