D.D.R

can monster discarded by the effect of D.D.R as its COST and removed from the game by other cards effect e.g. macro cosmos, be special summoned immediately? or it can only special summon a monster that's already removed from the game before D.D.R effect activate?

yes u can special summon the monster removed from play by cards like macro cosmo when u discard that monster as a cost to activate DDR. But do note that if u have no legal target at all before u activate DDR, then u cannot activate DDR at that point of time

its like when u activate divine wrath with devil king van'dragon effect

divine wrath cost = discard 1 card so u discard 1 monster that can be special summoned
after counter trap resolve devil king come down with the SPECIAL summon ability = revive the monster u throw...

same applies with DDR thats why DDR is GAY!!!