if you have legal parental responisibility, you will not lose it if someone else is also granted it. Multiple people can have parental responsibility.
Your wife does have the right to change you children's names by deed poll, but I believe she requires your consent also.
for the sake of an amicable agreement, perhaps the children could have their surname double barelled, i.e, their old and new name. This would help their sense of belonging and not turn into a feud between you and her. It is particularly important if she intends having subsequent children with her new partner who will then be their siblings as it helps to bond them together.
These relationships are never easy, but one thing your kids don't need is to feel as though their are sides to take.
If your wife died, you would be able to apply for a residency order. (not guarenteed that you will get it though.)
It is your job to ensure she gives you access. You need to see a solicitor and have it set in law when you are to see your children. If you do not do this and thus cannot take her back to court if she refuses, then the court could see you in a poor light for not being proactive. The courts will decide whether or not there is a reason why she is denying access and whether or not it is justifiable.