OYCR or OYCN

"Offer you can't refuse" or "Offer you can negotiate"?

Is it ever acceptable to say "no thanks" to a suggestion from "management" that you "consider" taking over one of the 3 leadership roles in the Dept in 9 months time, which will be 9 months after your promotion to full professor. This role would put me in charge of all the teaching management, and the management of all Faculty academic staff, as well as strategic decisions regarding the direction of the department. It currently takes up around 0.6 of someone's full time post. I only work 0.8 FTE (full time equivalent) so unless I negotiate it down, I will be spending a disporportionate part of the next 3 years being an administrator. I accept that at some point I will need to do this or a similar role, but I wasn't expecting it to be yet. I am also conscious that there has never been a female HoD in this department (nor a part-time one either). Do I:

a) say "no, I can't do it now" - with justification that I am too inexperienced to do the academic staff management, and it would have a disproportionate reduction on the amount of time I have for research?

b) say yes ok, I'll take part of the role, but 20% of it needs to be split out to other people (I would rather this were the academic staff management role as I will feel very underqualified to be doing reviews of senior professors in the department)

If b, what else would you negotiate? More money? Sabbatical at the end of 3 years? A female mentor (mentorship for leadership positions is dire in this university).