If you created a checkpoint immediately prior to adding mirror-2, you could theoretically just use the --rewind-to-checkpoint option when importing the pool. (You will lose everything created / modified after the time of the checkpoint’s creation.)
Otherwise, it’s looking like your options are:
- A “recovery” import, with TXG magic, if it’s even possible
- Use a tool for ZFS data recovery, such as Klennet
- Restore from your most recent backup