Will the deceased father's and the inheriting son's liability for zakat on gold bars be absolved if the son sells them and gives their value with the intention of general zakat, given that it is unknown whether the father paid their zakat in the last years of his life, and knowing that the bars remained with the father for 45 years and exceeded the nisab?

If your father's wealth reached the nisab (definition) (threshold) and he did not pay zakat (definition) for a period, then the zakat must be paid from his estate before its distribution, because zakat is a debt that takes precedence over inheritance. This applies if you are certain or highly assume that he did not pay the zakat. However, doubt does not obligate you to pay it. If you sell the ingots and pay their entire value as zakat, your father's obligation and your own will be cleared, even though the actual requirement is to ascertain the years during which the ingots reached the nisab and no zakat was paid for them.