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Yes, it's not like in the quarter-finals and semifinals where the matches were carried out in 2 meetings, away and home. If they lost the first game, they still had a chance to make a comeback in the second, now that scenario didn't exist. You're right that if they can't score, then they have to prevent the opponent from scoring too and take the game to penalties. In the penalty shootout round we can't be sure which club is the strongest, because for me the penalty shootout is a lucky shootout which gives the odds to both clubs being 50/50. There are many examples of favored clubs losing in penalty shootouts.
I'm not sure about the possibility of the final reaching penalties stage this season. I verry believe the winner will only be decided in the regular 90 minutes as both teams seem to have good chances to score throughout the match. Of course that's because both teams have reliable strikers, but if Inter defend completely and force the match into extra time then I think it's 50/50 to their chances.
So far I find it hard to imagine that Manchester City will fail this season in the Champions League. Their successes against Bayern Munich and Real Madrid mean they are more expected to win the title than Inter. Manchester City have proved they are better than Inter since the round of 16 with Leipzig, Bayern Munich and Real Madrid being the stronger teams. Meanwhile, Inter have much weaker opponents where Porto, Benfica and Milan are teams that tend to be not favorites in the Champions League compared to Bayern Munich and Real Madrid.
If Manchester City can secure the Champions League title, they will make an impossible record this season. Because this team has already secured the Premier League title. Hopefully, if they secure the Champions League title, this team will become much stronger and more expensive next season.