Help with leveraging a mistake to negotiate my salary

The long and short of the situation is a senior sales officer abruptly left after major changes were implemented. The department had a major restructuring and several low performers were let go.

I was immediately placed to be their replacement as my manager was already training me with the idea of expanding the team to keep pace with growth. My manager’s boss, one of the new C-Suite promotions said they approved of the promotion and I would be a great fit.

My boss knew our team was going to reach critical low levels due to major life events like major surgeries scheduled. The restructuring meant everyone has a greater share of work. My boss desperately needs to fill this position to have adequate staffing.

So it was a shock to everyone when my boss asked HR for the status of my promotion for nearly a month that our C-Suite leader never submitted the request and went on an international vacation where they can’t take any calls. I was actively covering for at least 3 people at a given time since the request was made.

I thought to cheer myself up I would wear a few suits I got to celebrate the promotion. My manager has always been an amazing leader but it was unsettling how keen they were on expressing how much my mental health and happiness was one of their top priorities. They finally jokingly asked in a nervous tone”…Are you currently interviewing?” It immediately clicked if I left it would crippled our department, one of the major profit centers of the entire organization.

The average starting base pay for my promotion is double my current salary. Currently the hiring process for this position if posted externally takes 3 months to recruit and 2-3 months of interviews due to the seniority of the position. The C-Suite leader comes back one week before multiple people on my team are scheduled to go on urgent medical PTO. They have no time to field another candidate and the last time several top candidates used their offer letters to leverage their current employers for better salaries meaning they are rolling the dice to hire externally.

Meanwhile my employee reviews are exceedingly high for a junior officer. My Organization published criteria requirements for certain senior positions and titles and I met a senior officer’s requirements. I volunteer at multiple organizations, I hold certifications from recognized organizations and developed a strong COI network.

I want to ask for $20k higher than the starting base especially now that I am unofficially holding the role until the C-Suite employee gets back. What is the best way to go about leveraging this particular situation? It seems HR gets the final say on compensation with recommendations from hiring managers and department heads.