Is it permissible for the questioner's husband to own the computer that her father gifted to him after its original owner refused to take it back and repair it at his own expense, or is it considered part of the inheritance, and thus the heirs have a right to it?

The aforementioned device is not part of the inheritance, and the heirs' right to it is forfeited because it was a gift from the device's owner to your brother. If your brother acknowledged the gift to your mother, it is valid. If he does not consent, then he has the choice between paying your brother the cost of repairs, or taking the value of the device on the day he received it from your mother and leaving it for your brother.