First of all, It is not permissible to use the sensor unless the use of normal methods of measuring glucose is critical for the patient, and the assessment of this criticality is up to the patient.
And if he is compelled to use the sensor, then he does not need to do ghusl as long as the sensor is present. Rather, it is sufficient for him to do tayammum. Then, he takes a bath when removing the sensor, which usually happens after two weeks, so he removes it and takes a bath (ghusl), then puts in the new sensor.
1- This ghusl is not acceptable, and it must be repeated after removing the sensor, as its function is tayammum instead of ghusl.
2- Tayammum instead of Friday ghusl is not sufficient for prayer. Hence, it is necessary to perform ablution.
If he prayed without ablution, these prayers have to be repeated.
3- Yes, he can.