Question

Is it safe to assume that:
generally, it is better to invest more on a Pokemon's HP (through EV training) than on its Def and Sp. Def, given that the Pokemon's base HP, Def, and Sp. Def stat are equal? (Moreover, it is hard to predict whether an opponent will use a Physical-based or a Special-based attack)

Answer 1

Well, basically, Defense and SD are useless without HP. Defense and SD block attacks better than HP, so ur best bet is to invest in def and SD. See This to help with investing evs: http://pokemondb.net/pokebase/14622/how-do-you-decide-on-evs-for-a-defensive-pokemon