With recent changes in UEFA there is some good news for Barça regardless of the decision. The change in card accumulation does not carry over into the Champions League final match. So if Messi were to pickup a yellow card in the next match and the next round he would not be banned from the final match if Barcelona were to advance. The only way a player could miss the final is if they picked up a red card in the semifinal match.
Article 21
Yellow and red cards
21.01 As a rule, a player who is sent off the field of play is suspended for the next
match in a UEFA club competition. The Control and Disciplinary Body is
entitled to augment this punishment. For serious offences the punishment
can be extended to all UEFA competition categories.
21.02 In case of repeated cautions, a player is suspended for the next competition
match:
a) after two cautions in two different qualifying matches, as well as after the
fourth caution during the qualifying phase;
b) from the first match in the group stage, after three cautions in three
different matches, as well as following any subsequent odd-numbered
caution (fifth, seventh, ninth, etc.).
21.03 Single yellow cards and pending suspensions are always carried forward
either to the next stage of the competition or to another club competition in
the current season.
21.04 Exceptionally, single cautions from the qualifying phase that have not
resulted in a suspension expire on completion of the qualifying phase in
question.
21.05 Cautions and pending yellow-card suspensions from club competition
matches expire at the end of the season.