Praise be to Allah.
It is permissible for your parents to go for Hajj if they pay the installment that is currently due. They do not have to wait until they have paid all installments.
If they are able to pay the current installment and are able to go for Hajj, then it is obligatory for them to do so. The same applies to ‘Umrah.
They also have to repent to Allah from the riba which they have taken upon themselves.
And Allah knows best.