How can I merge these two dataframes?

df1

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df2

Rule:

1. Merge based on callsign and re (if not null). If de and d_runway do not match, add a label indicating a mismatch.
2. Merge based on re if callsign is null.
3. If both callsign and re are null, merge based on de and d_runway.

There is also a timestamp that plays a key role in merging such that each dataframe has a timestamp and can only be matched if the difference is within 900 seconds

Output