Urgent!! - Reject Job offer after receiving the BRP and before joining date.

Today 4th Jun 2021
I'm currently working on ICT visa.

2nd May 2021: Company A raised super priority visa (skilled worker-Switching immigration category) with start date in COS as 28th Jun 2021.

Company A hiring team didn't respond to my emails, So I proceeded with Company B

2nd June 2021 Company B(fully aware of my situation) had raised super priority visa(skilled worker-Changes of Employment - ISC liable) basing new skilled worker BRP given by company A. I have visa appointment on 26th-june-2021.

Company A responded back on 4th June 2021 asking for expected joining date. I'm very sceptical about them on honouring job offer

Queries:
If I had to reject Company A's Job offer.
1) Can Company B raise visa (skilled worker-Changes of Employment - ISC liable ) basing BRP of company A before joining date?

2) If I reject Company A's Job offer before visa appointment, will it affect my visa status and company B's visa application ? When should I reject Company A's Job offer?

3) If Company A gets annoyed, will they do something under immigration law? will it affect my visa application ?

4) If Company A gets annoyed, will they do something under employment law? will it affect my visa application ?

If I had to reject Company B's Job offer.
5) Since my visa application with company B is started, will that means I can no longer work with Company A?

Thanks a lot for the patience.

Cheers