1: Is it allowed to swim during the mustahab fasting?
2: Can we break the fast if someone is offering you food even though the person knows I am fasting?
3: Is there a time limit to break the mustahab fast or it can be at anytime if anyone offers food that we can consume it?

1. According to Ayatollah Sistani it is allowed but strongly discouraged
2. If it is recommended fast and not obligatory, yes you can break fast
3. It can be anytime