Council of Elrond

Unique. The following effect applies only if the bearer is an Elf Warrior. In company versus company combat, make a roll (draw a #) before strikes are assigned for each non-unique minion ally in the company the bearer is facing. If the result for an ally is greater than the ally's mind plus five, discard the ally.

If a FW company is overt through Regiment of Black Crows and initiates CvCC against my hero company, then if Bow of the Galadhrim discards Regiment, the FW company is no longer overt. Is CvCC allowed to proceed in this case?

I don't know any rule directly concerning this question.

However one that says about similar situation.

CRF, Turn Sequence, Site Phase, Automatic-attacks wrote:Company composition and overt/covert status is checked before each automatic attack.

So in middle of attack status of company is not checked. If conditions of declaring attack would be checked after attack is declared then changing site path/site type would cancel attack from successfully resolved creature too.