lisa firstly unless and until your employer have your notice in writing, they must work on the assumption you are coming back from maternity leave.
If you are 'on holiday' you are 'back at work', so that would 'count' towards the 13 weeks' service you need. I suspect your boss is trying it on a bit and hasn't spoken to HR, but it's difficult to see what she would gain out of saying what she's said, other than having your notice in writing would allow her to go ahead and appoint this person on a permanent basis to your job.
There is no obligation on the part of the employer to allow you to take your annual leave straight after your maternity leave in this way though. Your boss would be perfectly within her rights to refuse a holiday request like this (and a holiday request for 13 weeks of holiday in one go is a hefty request), leaving you to decide whether to either return to work for 13 weeks or resign.
The other thing I would query is the bit about you and HR working out between you that if you 'go back on 18 hours', you can use your leave in that way, etc. Does going back on 18 hours represent a change to your normal working pattern? If so, and you and HR have worked all this out between you - your hours and your holiday and your boss hasn't been involved, she is probably a bit put out and I wouldn't blame her.
If 18 hours would be a change to your normal hours, you should put it in writing in the form of a flexible working request, or whatever policy you might have, and get it signed off. You may have no intention of actually physically returning to work but if you are hoping to change your hours and be 'on holiday' for 13 weeks, everyone will need to work on the assumption you are coming back until you hand in your notice 4 weeks (or however long your notice period is) before you want to actually leave.
I hope you can sort this out so you get your money. But be aware there is no obligation on your boss to agree to this, and also think about the principle of the policy about extra pay. The extra 12 weeks at half pay is there as an incentive for women to go back to work. You are not doing that, you are looking for a way round the rules to get some money. I don't at all blame you for that, but if it doesn't work out, try not to feel too hard done by.