1) Can infantry grant a vehicle a 5+ cover save if they obscure 25% or more of the targeted facing of a shooting attack?
Yes, if the model being shot at has at LEAST 25% of its hull being blocked by LoS stuffs (Rocks, Trees, Trygons...) it gains a cover save.
2) Page 75 of the BRB specifically mentions that vehicles do not automatically gain a cover save from being inside of area terrain...
i was pretty certain that if you are in area terrain you get the cover save...at least for things like forests & ruins...
I also believe that if you are shooting through...i think its more than 2", but this might be a 5th ed rule... of area terrain (again trees/ruins) that they are deemed to be shooting through cover, thus granting you at least a 5+ cover save (since ruins are 4+ you would gain a 4+ cover save)
3) In a similar vein to question 2, what about shots fired through forests as outlined on page 102 of the BRB ("Cover Saves")?
it grants what whatever you are shooting through's cover value, so if its forests its 5+, ruins 4+, fortified ruins 3+, etc.
4) Related to 3: the aforementioned cover save is applied when "if one or more models in a shooting unit have to trace their line of sight through a forest (because they're shooting at a model beyond the forest)". Does this mean their line of site has to go in through one border side and out the other? Or does a firer within the boundaries of a forest grant his target a 5+ cover save simply by shooting out?
i believe that i answered this already.