You are reading too much into it and applying conditions that do not apply to you.
The authority is the form 8615 instructions themselves which say:
[quote]
Who Must File
Form 8615 must be filed for anyone who meets all of the
following conditions.
1. You had more than $2,100 of unearned income.
2. You are required to file a tax return.
3. You were either:
a. Under age 18 at the end of 2018,
b. Age 18 at the end of 2018 and didn’t have earned income
that was more than half of your support, or
c. A full-time student at least age 19 and under age 24 at the
end of 2018 and didn’t have earned income that was more than
half of your support.
(Earned income is defined later. Support is defined below.)
4. At least one of your parents was alive at the end of 2018.
5. You don’t file a joint return for 2018.
These rules apply if you’re legally adopted and a stepchild.
These rules also apply whether or not you’re a dependent.
These rules don't apply if neither of your parents were living at
the end of the year.
[end quote]
If you meet those requirements then you are required to file the form with your tax return. Also note that any reference to "custodial parent" does not apply to anyone that is over the age of 18 in any state where age18 is the age of emancipation - after that there is not such thing as a "custodial parent" - just a parent, but you can be required to file the form up to age 23 if the conditions apply. Also note that any reference to dependent, or living with a parent also does not apply if you are not a dependent on a parents tax return. As it says, the only case where the rules do not apply is if neither parent is living.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
