Hello Marina!
You asked: “I have read that the gandhabba cannot bear the energy of the arahant. So what happens to the gandhabba when the ordinary living being becomes an arahant? Is it weakened and reduced by starvation?”
Gandhabba of an Arahant cannot bear the energy of an Arahant when OUTSIDE the physical body. So, when the physical body of the Arahant dies, the gandhabba comes out and perished.
– Gandhabba, in this case, can be compared to a heater coil. As long as the heater coil (with heat on) is immersed in water, it will be fine. But as soon as it is taken out of the water, it burns.
You asked: “And as the arahant passes into Nibbana, does the weakened gandhabba dissolve or is somehow reabsorbed into the mortal remains of the arahant?”
Parinibbana (attained at the death of an Arahant) and “this world” are mutually exclusive, meaning that one can either in this world or in Parinibbana.
– Parinibbana means “complete and full Nibbana.” Until then an Arahant has Nibbana but still has the physical body living in this world.
– So, at the death of an Arahant, there is nothing remaining about him/her in this world. That is why the Buddha likened that to the flame of an oil lamp extinguished.
You asked: “Why is it necessary to be ordained to attain Nibbana.”
It is not necessary to be ordained to attain Nibbana. One can stay as a “householder” or a ‘layperson” all the way up to the Arahanthood.