I am on a tier 2 general visa and in dire need of your wise insights

I am intending on changing jobs and my prospective new employer has applied for a CoS to be allocated to me without consulting me on the start and end dates. Following similar other quite unprofessional behavior, I have now strong doubts about whether I want to work for this new company.
A couple of related questions to which I would appreciate your educated answers:
1) Is the start date on the CoS (I understand that it can be amended after issue) the exact one which will show up on the tier 2 visa ? Or can the tier 2 visa start date be different?
2) In case I decide to stay put with my current employer, Is it safe to cancel the new to be issued CoS so that I can still continue to work for my current employer without them having knowledge of my previous intentions ? This wont affect a future application for an ILR?
3) Tricky one: I have seen a LOT of contradictory comments on this one including from immigration consultants so PLEASE only answer if you are really certain or have actually gone through this: My wife is my tier 2 dependent, does she have also to apply for a new visa/BRP if I change my employer?
4) Once a tier 2 visa is issued. Can its start date be brought forward (brought earlier) ?
Context: My new employer wants me to start asap after I get issued a new visa but I have 3 months notice with my current employer. While i plan to negotiate down the notice to 1 month, I have no way to make sure that I can do so. This is while the tier 2 visa start date has to be agreed upon before the visa application and before me giving my notice. Other brighter solutions welcome.
Thanks !!
Alex