UK Visitor Visa multiple sponsorship - requirements of sponsors' invitations to stay, and UK citizen but not UK resident sponsor

I (British) and my partner (Ethiopian) live in Ethiopia and are applying for a visitor visa to visit my family in the UK.

I have assumed I would be the sponsor, and that I would get invitation letters from my mother and father in the UK.

[EDIT: For anyone reading through this, it seems this form below is outdated and no longer necessary for a visitor visa.]

However the Sponsorship Undertaking form SU07/12 states "The sponsor giving this undertaking must be resident in the United Kingdom" (although it provides separate boxes for applicant's accommodation address and sponsor's address).

All my bills go to my mum's in the UK, I am registered to postal vote, and my citizenship is UK, but I am tax resident in Ethiopia, as I live and work in Ethiopia and only visit the UK occasionally. (The only qualifying document I might have confirming my UK base at mum's is a letter regarding my overseas vote registration.)

Do I have to get my mother to be the sponsor, as she is normally resident in the UK?

Is there therefore no point in me submitting my bank statements, given my mother is taking legal responsibility? Or should I be an additional sponsor (but not fill in SU07 due to being normally resident in Ethiopia)? My mother and I have ample savings to cover the trip, although obviously I'd be paying them. Part of our application is my having covered our expenses for holidays to Asia and Africa over the last 3 years of our relationship.

My mother and father live apart and we hope to stay with each of them for periods during our visit. Do the dates on their letters need to match the trip itinerary and not overlap? Or can they both send invitation letters covering the whole trip?

We are hoping to travel in 6 weeks, with Christmas breaks in the middle. Should the invitation letters extend past the trip end date, in case the visa application is delayed and we have to travel later?

Do my mother and father both need to sign SU07 and submit proof of mortgage? If yes, my father is travelling so I can't get an original signature (I can get a bill from his home in the UK, but I have no-one to bring a signed form from him in Vietnam to me). Would it be simpler to just stay at my mother's? (They live a couple hours apart so this would not stop us spending time with my father.)

Does this make this application a 'complex case' that you'd recommend I hire a solicitor to help with?

Oddly, the guidance lists "sponsor’s utility bills" under "Documents you should not send unless specifically requested", (my emphasis), while form SU07/12 says "The sponsor should provide evidence that he/she lives at the address given above", one acceptable form of which is "Bill from a utility company (gas, water etc.)". I will get another form of proof, but if anyone has had any feedback on this it would enrich the answer.

Enormous thanks. Apologies if I've missed guidance or a relevant question on here.