Tier2 ICT to Skilled Worker Visa

I would need assistance to file my application and get answers to below queries before I can get the COS generated from the new employer.

1. When should I resign from my current company - after getting offer letter, after getting COS, after getting BRP?
2. What should be my ideal COS start working date considering my current visa expires on 10 April and need to serve 90 days of notice period? This is to the point I would like to file my ILR and want to get the visa once for 5 years straight. 
3. Can I join the new company earlier than COS start date if my visa is curtailed and visa is under processing?
4. What is the maximum number of days I can stay in UK without a job if the current employment ceases on 14 April and the next employment joining date is in future?
5. Can the future employer prepone/ postpone the joining date for the job which would be different from the one mentioned on the BRP?
6. Can the future employer prepone the joining date for the job, if the current employment job is over and the new application of SW is still under processing?
7. Can I continue to work for my current employer beyond 10 April if my new job BRP has start date of 20 April?
8. If the current company cancels the visa immediately after resignation, what is the impact on my status in the UK as the application might be under process or I would have a BRP to start my next job in the future date?
9. If I keep the COS work date for the new employer as 21 April, and my current employer visa expires on 14 April. Can I continue to work for my current employer from 10-20 April and join my new employer on 21 April even if visa is under process/ received the new BRP?

A long thread with too many questions puts off many from responding.

I will try answering the ones I can.

1. After getting new BRP

2. Not entirely sure. 11th April if you need to serve 90 days notice?

3. Not everybody is eligible to do so. See section 'If you’ve applied for a work visa and are waiting for a decision on your application' under:

https://www.gov.uk/guidance/coronavirus ... -residents

4. There is no employment gap rule but you need to apply for your new visa before the current one runs out

5. Unsure. You need to check with your new employer's HR/Immigration dept who have access to UK visa sponsorship management system

6. Same as 5

7. Yes, only if you applied for your new visa before expiry of current one

8. Employers cannot cancel visas/BRPs. Only Home Office can. As long as you apply before your current visa runs out you should be fine.

9. Same as 7

Thanks for the help!