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If the partition lasts less than a finality period (12 hours currently IIRC) then when remerged, then the effect is simply that one branch will become mostly red. Since red block's transactions are included in the sequencing, the effect is non grave and non conflicting txs will be confirmed.

If a finality period has surpassed then at least one of the parties should reject that of the other and not be able to naturally remerge. Resolving of the partition would require manual intervention and one of the sides accepting the other's as "the truth".

Once resolved, merchants should rebroadcast transactions. Default transactions are not time dependent and can be rebroadcast as they are. I don't think time expiration is a thing in txs at all, though I guess it could have been.