>>>The ramban doesn’t say that the mitzriim did so independently; he aays that they went beyond the gezerah of avdus, but there obviously is a new gezerah,
Where did you see that in the Ramban? He seems to pretty clearly say otherwise
וזה טעם דן אנכי שאביא אותם במשפט אם עשו כנגזר
עליהם או הוסיפו להרע להם
seems to be clearly saying that they added on the gezera on their own. If there was a new gezara then what does הוסיפו להרע over the gezeira mean?
I’m not going into the whole question of whether the Ratzon Hashem that (a) should have bechira would allow him to harm (b) even when (b) wouldn’t have otherwise been harmed is a possibility or not. I’m well aware that there are different views on this and the sifrei musser take the view that the answer is no. You however seem to believe that saying the answer is yes is kefira. What are your sources for that?