First of all Tier 2 route does not exist anymore so you will be applying for ILR under Skilled Worker rules for ILR.
1. There is no specific format. This is what your employer must provide:
indefinite-leave-to-remain/indefinite-l ... l#p2019979
2. The application form that asks you for all periods of absence. You also should submit passport stamps in the last 5 years. Also, note that PBS dependants whose visa was issued before 11 Jan 2018 do NOT have absence limits.
3. There is no specific requirement for this under the rules but as a general rule,
ANY evidence from
ANY source can be used for cohabitation. Covering the last two years from at least 3 different sources. Use bills, agreements, council tax, letters, etc
Advice is given based on my personal research and experience only. Do NOT contact me via private message for immigration advice