I would indeed combine the entries and then send the 1099-B to the IRS and let them sort the mess out. If you have made your good faith effort to break the transaction up as much as you are able to do and pay the taxes owed on what you have come up with then you have done what you can and if they have an argument let them make it.
You can combine the 18 ESPP dates with a 'various' purchase date. That's what I would do.
@amvivaldi
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