If person A has a DCFSA but does not claim a qualifying dependent on their tax return, the amount will be added back to their taxable income. There is no additional penalty for this situation and the add-back will be done automatically by Turbotax.
If person B claims a qualifying dependent and files as single or HOH, person B can have a DCFSA of up to $5000. The issue is whether your employer will allow you to change the amount (they probably won't or can't allow it unless you are still within the 60 day eligible window from a qualifying life event).
If your income is more than $15,000, you could never allow your unmarried partner to claim your biological child as a dependent, even if you wanted to.
There is no issue with FICA.
