Not if the other parent has joint custody and/or visitation rights.
In general, yes. The payments are for the children, not the parent. The step-parent is not legally responsible for the children unless he adopts them.
Why should adult children have any rights to the personal gains of a parent, unless they worked to add to them?
No in most states, and even than, the separated parent can file an injunction to prevent it or have the children returned.
It depends if there was a will or not. Step-children have no rights to the estate of a step-parent unless specifically named in the will. If there was no will, the estate is typically divided between the spouse and the children. Check the laws for your state or jurisdiction.
Not as a reason to obtain a modification, however it can be a consideration at the new automatic review, or if the obligee parent files for an increase. see link below
No. You cannot do anything to interfere with the other parent's access to your children. You need to petition the court for permission to relocate.
Becoming a parent earlier, getting divorced and remarried more.
It can be modified, but not stopped.
Yes he also has had 2 children