You declare any additional life point expediture or other bonus increases before you make an attack check this includes any increases to damage from advantage cards or other sources. This is so your opponent can make an informed descision on how they will defend themselves (Such as Dodging with Gnosis or using another special reactive ability).
Usually the only time an advantage card is revealed after an attack is concluded is if the card states that in such an event it should appear. OF the top of my head I could only see Dragon's Blood the special promo card falling into that category.
As for modifiers. All sources of modifiers stack in Anima,
e.g. Lorenzo Charges an enemy model (Kronen) using his special charge he spends an action point for +2, Sophia, allied with Lorenzo reactively activates her abilities giving a further +2. Lorenzo charges and adds in that modifier for a grand bonus of +6.
Kronen uses his Special Ability: Scarlet Heartbeat and gets lucky rolling well on his defense and avoiding Lorenzos attack. In turn Kronen may counterattack which suffers an innate -2 to the attack but Kronens ability gives him +4 resulting in a total bonus to hit of +2