DNP or MSN-administration??

I think this will depend on potential employers and whether they find that your DNP degree will fulfill the requirements for the administrative role. I think that many would find it sufficient since you likely had the same core courses as an administrative track when completing your degree. However, some will probably want the business specific courses in the certificate program. Sorry I'm not much help, but I think putting out a couple applications, or contacting someone you know that works in administration and has an open position, may give you a better idea of what they're specifically looking for. Good luck!

Honestly, forget the MSN in administration. Get an MBA or an MHA. They're very translatable, well understood, and won't necessarily pidgeon hole you in nursing administration. You'll also get a broader business education than the MSN. You could take leadership roles in other areas where they won't struggle to connect the dots. For example, I have a colleague who is admin director of surgical services. She's an RN, but managed the finances of the department, managed all administrative staff, NP/PA staff from admin side, and just generally runs the department save for medical policy (MDs have separate reporting structure).

Knowing these folks, they might have struggled to understand how a Nursing admin degree might have translated well since there are no nurses within the department. I just think these two degrees will offer more than the MSN, not to mention you have an MSN anyway. 

I think it really depends on what kind of administrative role you would be looking for. I am currently enrolled in a Nursing Admin MSN program because I am really only interested in an administrative position on the nursing side of things... I have no interest in getting a more business-focused degree and managing finances. But I could definitely see how an MBA or MHA would be beneficial since you will already be an FNP.