Jointly held property

We act on behalf of numerous married clients who have held let property as ‘joint tenants’ and our understanding is that income is split 50:50. However we are aware of – and a little confused by – various articles advising on different allocations as follows.

Can the property be split as tenants in common (say 90:10) by a legal transfer and what evidence is needed to prove this beneficial ownership; for example is a simple deed of trust sufficient?

Does a form 17 need to be completed as well as the legal transfer above?

Presumably ownership shares can be altered during lifetime (e.g. back to 50:50) if tax status changes; however would this be queried?

If the property is held jointly other than by a married couple (e.g. by parents and husband/wife) can the income be allocated differently without a change in legal ownership; for...