Hello,
I have not yet tested this, but would the following be possible with balbooa forms:
a) Upon register, run custom PHP in order to populate a custom table (without a joomla table prefix)
b) Populate form fields with a custom query upon prepare of form in order to update a specific data set
c) Fire custom error message for specific fields or form (validation of fields)
d) Add custom fields (e.g. date of birth) with register form
Since you have updated forms by offering a register and login form, I am not sure if those will be flexible enough in order to run some custom code. I am currently using Converforms (flexible), but I would like to use Balbooa forms in the future.