Orinoco what you're saying is quite contradictory.
In terms of what you are doing, it does sound as though a review of your job description and salary is a reasonable request. A change of hours is a different request altogether, (although it might help finance the first request.)
But if you've been running the office by yourself with no support, overseeing huge projects and having meetings, are you really in a position to reduce your hours? If your job is much bigger than it is supposed to be, you will need to come up with an excellent proposal about how you think it can be done in less hours, even though it's more than you should be doing already.
Explaining how you need a regrading/pay rise because you are doing loads more than your job description and then at the same time saying you can fit everything you are doing into less hours is a tricky proposal to put forward! Think carefully about what you are saying. With so much going on, are you proposing some of the more junior stuff gets done by someone else? Or can tasks be done differently/more efficiently/don't need to be done at all?
I would suggest you write your own job description, write what you are actually doing. Then look at it realistically and think whether it can be done in less hours. Think about how this would work. It might involve coming up with a wider proposal about how the office is run. Then put forward a proposal including a rewritten job description for you, details of what hours you want to work, details of how other work will be covered, and outlining how all this will benefit the business.
For more on how flexible working can benefit the business, have a look at working families website, there's an online guide and some factsheets that might be useful.