Your business structure directly impacts how lenders assess your income. A sole trader earning $150,000 profit will be assessed differently from a company director paying themselves a $90,000 salary with $60,000 in retained company profits.
You can, but it's generally not advisable close to a mortgage application. Lenders want to see 2 years of consistent trading under the same structure.
It varies significantly. Some lenders add 100% of retained profits. Others only count 50%, and some ignore retained profits entirely. A broker experienced with self-employed lending can match you to the right lender.
Neither inherently — it depends on how income is distributed. Consistent distributions to you each year are assessable. Wildly varying distributions complicate applications.