I would rule that a player is not allowed to invoke aspects at any time they want to in a conflict. By this I mean, without any real support in the rules, that one side first invokes all the aspects that he or she wants, and then locks the results. After that the other side does the same. In case of an attack, the attacker starts, and then locks the results. After that the defender rolls and locks the results. A conflict can be seen, from the “attackers” point-of-view, as an apposed roll without knowing the target number (i.e. the defense roll has not been made at the time of the attack). I would use the same way of thinking in case the player would roll against a fixed, but unknown, target number. Once the player have made the roll and I reveal the target number, the player would not be allowed to suddenly invoke an aspect after realizing that he failed the roll and re-rolling or add +2 to the result.
Please also note that once you have invoked an aspect to add +2, you are not allowed to re-roll, as the rules say that “Add two (+2) to the final die roll (after any rerolls have been done)”.