But paying the big bucks for the professionals to re-mediate the mold is not necessary.

It just might mean that you have to go through mediation/arbitration and/or small claims court to do so.

For the future, I would recommend that you put in your lease contract that the tenant agrees, in an event of a dispute to resort to arbitration /mediation.

(Maybe i did unknowingly) I brought in my trusted realtor who became my mediator.

Outside of the direct cost to re-mediate, would love any feedback on how prior mold issues can affect the resale value on a flip? 

I'm not afraid to call them out, Max Maxwell & Sean Terry in particular comes to mind, who I think popularized and and social media-tized the business and made it seem easier than it is.

At this point, I would be content with mediation and having the HOA just wave the fine to prevent me from selling the unit.

You have mediation in the listing agreement?

Our state has a free support line for all landlords when we have questions, and they have referrals to attorneys and other providers like mediators.

California adopted a new Partition of Real Property Act effective 1/1/23 that generally makes it easier to force sale and distribution or buy out by appraisal when faced with uncooperative co-owners.It is possible that if you get a lawyer, your Dad's former partner might decide to be more reasonable as an alternative to litigation, or a satisfactory solution might be achieved through mediation.