Is it permissible for my grandmother to pray while she is unable to perform ablution, and there is some impurity in her diaper, given the difficulty of changing it for every prayer?

If the stroke caused the grandmother to lose her consciousness and awareness, then prayer is no longer obligatory for her. If she did not lose consciousness, then prayer is still obligatory for her, and she must perform ablution (wudu') if she is able; otherwise, she should perform tayammum (dry ablution). She should pray sitting if she is unable to stand, and she is required to purify herself from impurity. If purification is difficult or impossible, she should pray in her current state. She may combine two prayers with full purification if that is preferable to praying each prayer at its prescribed time with incomplete purification.