It's an incredibly hard situation OP. Look after yourself, as well as your mum. When we went through this, it was a combination of dealing with our own anticipatory grief and shock, trying to look after my parent and advocate for them, and dealing with practicalities like where paperwork is (will and executor details, pensions, passport, bank account, address books, utility company, house deeds etc). We just tried to spend time together too.
My dad had pre-paid/planned his funeral and left a lot of detail to help us organise it, which really helped. Whilst it's sad and something that's very hard to think about, it might be a good idea to start noting down her wishes for the funeral now. It will make it far easier when the time comes as you won't need to 'guess' what her preferences might be. It will help, if you already know which funeral director will you use so you can easily call them, especially if she is planning to return home for her final days. They'll guide you through the funeral planning and the initial paperwork needed by the registry office. What music, songs, hymns, poems, flowers, photos etc does your mum like? For you, if there will be a eulogy, what do you want to include? What are your mum's best memories / most significant experiences? What does she want to be dressed in? Who are/were the most significant people in her life and who will you need to contact (and how, does she have an address book)?
If she has time to go home, have you been put in touch with your local hospice and Macmillan nurse? Will your mum need carers? Will your GP put you in touch with the local district nurse team and let you order repeat prescriptions for her, as needed? Our local hospice was fabulous. They gave my dad quicker access to OT, physio and hosted activity days that he went to for a while. The Macmillan nurse visited regularly. She talked to dad and gave him an outlet for questions he didn't feel he could ask anyone else. The district nurses visited a few times a day to check dressings and to check on pain relief, including in the care home he moved to for his final days. There was information online (one of the main cancer charities) describing what happens as people die of various different cancers and what the terminal slide is like.
If she has time you might need to look at registering a power of attorney for a few family members for finance/medical and putting these in place with the bank/GP. (My dad had made his bank account a joint account with my mum (his ex-wife) so we could use his funds to pay for the funeral as the account automatically transferred into her name when he died and it took a while to get the will registered.)
The well known online shop with super fast delivery service and our local 24 hr supermarket were a real help. We could quickly buy what my parent needed to make him more comfortable (clothes, cushions etc).
My heart goes out to you, OP. It's a tough situation for you all. Hugs xx