First, separate compensation issues from the practical issues they face. The two don't mix well.

Second, Norwegian will not reimburse the cost of the rental or the self-help ticket.

Third, your parents are likely stuck in steerage, but they can make the argument for J. It is possible that J is not being operated at all on the flight.

Fourth, if they do fly the alternate carrier, they do need to make certain that their return is preserved.

As to restitution, Norwegian is on the hook for compensation of EUR 600 per passenger for the delay (presuming that they will be delayed by 4 or more hours, EUR 300 is 3+ hours). It is also on the hook for a reimbursement (not compensation) of 75% of the segment fare for the downgrade (not just the fare difference). But, it will take forever to collect this and given Norwegian's financial stability, there is a good chance that they will have a claim for these amounts which will never be paid.