Hart- I think you are trying to logically work your way through this situation but you need to trust the experienced parents on this thread- don’t make up your own rules, don’t try to confuse the issue with extraneous facts, and DON’T make this harder than it needs to be.
If you live most of the time (or more than 50% of the time) you use your mom’s financials. Period, end of story.
The fact that your father has sole custody (not sure I understand a custody arrangement where you don’t live with this legal custodial parent but that is totally irrelevant for filing FAFSA) doesn’t matter.
Your mom’s info goes on the form. She worked and filed taxes in 2017. The FAFSA form will determine if you qualify for simplified means- again, you don’t get to pick and choose what information goes where.
OK?