@OneJadeSheep you are perfectly entitled to work part time, it is legally permitted by legislation and contractually permitted by your employer. What they are doing is bullying.
You said your job is Global Head of Accessibility and that this is a Global business. Use your position wisely.
Realistically going from 5 days of 7.5 hours for 46 weeks a year to 2 days of 6 hours for 30 ish weeks a year is a big change. No one can do a full time job in part time part year hours. The ideal position is to make the role a job share.
I would tackle this strategically short, medium and long term.
Short Term: Talk to a relatively senior trusted HR colleague - one who’s pragmatic and ask about the bullying and harassment policy. https://www.acas.org.uk/bullying-at-work Explain what is happening and use the policy to formulate a plan to address this.
As managers are involved I would do a training session on bullying and harassment using a number of examples and including a scenario similar to yours. Explain they have a responsibility to address bullying and harassment by them and their staff. At the same meeting explain your long term plan of a business proposal to increase flexibility as part of accessibility strategy including job share - this is the carrot!
Short to Medium term: Realistically you cannot do your full time job part time part year, no one can; but there is clearly a business need for someone to do specific parts of your role on your non-working days. Targeted delegation is the only way to address this. You need to build a structure to allow the business to continue when you are not there. Sell it as a development opportunity - yes everyone knows this is bullshit for more work but it’s the only option you have.
Note: If you work 37.5 hours 46 weeks after BH and leave it’s 1747 hours a year 12 hours a week for 30 weeks is 360 hours the missing 1387 hours have to be covered some other way. Being honest about it doesn’t stop it being shit for everyone but acknowledging it may help.
Long term:You have said your employer works on headcount rather than Full Time Equivalent so won’t agree to job share. Strategically as Global Head of Accessibility you are in the prime position to put forward a business proposal that flexible working, including job share, is a reasonable adjustment for disabled employees and can benefit the business. A barrier is the organisations approach to headcount rather than FTE. Using FTE does have a cost to the business and you need to include those costs in your business plan but there are also savings like retaining knowledge, loyalty etc.
Is it feasible you could delegate this?
I really admire what you are taking on an keeping working is important but your employer needs to be fair to your colleagues too.