Fiancé Visa Savings Account Holder

I have read the immigration rules and there is only one sentence that states the funds can be held by 'applicant, sponsor or both jointly'. I take it that for entry clearance fiancé visa, the sponsor can use their individual account in the UK and the applicant can use their individual account abroad and we can state the total is £88500 combined? Or does it need to be a 'joint' account?

How in detail do we need to be with declaring the source of the money? A large portion of the sponsor's money came from when they worked abroad, some from family etc. Do I provide dates and screenshots of the money leaving one account and being paid in another throughout the years or can it be a statement explaining the family gift is irrevocable and that other money is from work abroad?

Whilst I have scrolled through these immigration boards and seeing similar questions, they're not specific enough to use as evidence for my solicitor who is wary of the applicant using their savings for entry clearance. Thanks in advance!