Advice on add on booking

[Debbyt28]

Hi not sure if anyone knows the answer to this but here goes
I have booked a solo trip with BA at the dolphin . Coincidentally DD1 has been booked on a training course in Washington at the same time, she has seized the chance to gatecrash her old mums luxury trip she has already booked her internal flight and intends to stay two nights with me at the weekend and then return to Washington ( yes we are slightly addicted to Disney)
I have contacted the dolphin to see if she could be added to my room but they say as I booked with BA I have to go through them
So I contacted BA what a nightmare they said they would have to rearrange my booking they don't see a problem in her staying as I have in theory paid for a two bed room i.e. single supplement which I agree however we plan to visit 3 parks in one day the 1st being Hollywood studios and as its early entry we don't think she will be allowed EE if it isn't linked to her ticket if she's not officially a guest
I am hoping I can sort when I arrive at the hotel which will be 5 days before her but has anyone got any ideas
All advice gratefully received