Can you explain me why you are talking about dynamis things to answer to the question "did you test the elemental debuffs with 200+ INT?"
"Since when do you use those spells ?" well if you were a good blm, on all NMs... Actually the correct question is when do you not use those spells and the anwser is when you solo/need to sleep (even if we can break dot'd mobs now) a mob.
Back to the topic : The easiest way to test it is to do a ballista with a friend. I wanted to do it myself but atm i'm missing 3 INT to have 200 (missing blm af 3 hands). You can also try in abyssea with a mob which can charm like Raskovnik (but i dont have a real mvt speed so i will wait the af 3 hands xD)
About bio, since the spell is based on dark magic skill and since they said they uncaped dark magic, we have new tiers for sure but idk if the blm bio 2 is better than the rdm bio 3 with a capped dark magic skill + gear. Anyway go with full skill when you cast it. Your bio 2 will not overwrite rdm's bio 3 if he did it before you, and the bio 3 from rdm will always overwrite your bio if you do it before him xD
EDIT : I guess the only way to have a msg "no effect" with an elemental dot is not to have a less INT or something like that but ONLY if you try to cast the weak dot from an element already casted. IE : your friend landed a Frost, you will have this message if you cast a Choke