Does the whisper of misgivings regarding God's essence affect one's ability to utter the Shahada at the time of death?

You must resist the intrusive thoughts related to the essence of Allah and strive to get rid of them, for they do no harm as long as they are disliked, and their repeller will be rewarded. As for the fear of not being able to utter the Shahada at the time of death, this is a good thing, and it must be coupled with hope in the vastness of Allah's mercy and a good opinion of Him. This will motivate one to strive in worship and rectify one's deeds, and that will be a cause for a good ending.