A person with type 1 diabetes is using a sensor inserted into the arm. He does tayammum before any ghusl.
1. Can he perform tayammum after ghusl in case he forgets to do it before?
2. In case of ghusle jummah, can he do tayammum and ghusl and perform obligatory prayers without wudhu?
3. He uses sand from the beach for tayammum, Can he remove excess sand from the face and hands after the tayammum is finished?

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.