Is it permissible for my grandmother, who cannot control her urine and finds it difficult to go to the bathroom and perform ablution in bed, to wear a diaper and remove it at the end of the day to perform her prayers?

If urine exits your grandmother involuntarily, even once daily, she is not required to perform istinja' (ritual cleansing) from it according to the Maliki school. She is permitted to wear a diaper and remove it at the end of the day. However, if she is able to control the expulsion of urine, she is not obligated to go to the restroom; rather, she can use a vessel in her room, perform istinja', and pray in clean clothes without needing a diaper. She must perform wudu' (ablution) for each prayer as long as she is able. She can perform a "formal" jam' (combining) between two prayers by performing wudu' at the end of the Dhuhr prayer's time and praying Dhuhr, then praying Asr with the same wudu'. If she has urinary incontinence (salasil) or is ill, she has a concession to combine prayers according to the Hanbali school.