I do not know your manager or your HR Rep, so I have no way of knowing how trustworthy they are. There's no way I can tell you for certain that they will or will not keep their promise to honor the future date of your resignation or to keep your benefits in place until the date that is convenient for you. Since you have already given them unequivocal notice that you will not be returning, they are not required to continue your benefits.
What I can tell you is that in the absolute worst case scenario you will be eligible for COBRA.

