The parent who provided the most financial support to the student should fill out FAFSA. So, first question is did the parents provide an equal amount of financial support last year? You don’t get to the FAFSA guidance in your post if that’s not the case.
If they provided equal financial support, follow the directions that you posted and choose the parent with the higher income and assets. In your example that would be the parent with $50K income and $500K assets (I’m not sure what you mean when you write ‘equity’…you want total assets.) FAFSA doesn’t really give any more guidance than what you posted, so just be able to support the decision you make.
Note that the trust where your kid is a beneficiary (that you asked about in another thread) is attached to your kid, not the parent. So that will be on FAFSA regardless who the FAFSA contributor is.
Lastly, and I think you already know this, most CSS profile schools will require the financial info of both parents (and any new spouses.) Below is a link to the schools that use the CSS Profile. You can see which ones don’t require the non-custodial (that’s what they still call it) parent’s financial info in the appropriate field. https://profile.collegeboard.org/profile/ppi/participatinginstitutions.aspx
@kelsmom anything else?