Is my husband, with these characteristics, acceptable to God, or has his failure to pray in his last days due to the severity of his illness and psychological condition nullified his previous deeds?

We ask Allah for mercy and forgiveness for him. What you mentioned about his state gives hope for the acceptance of his good deeds and salvation, but we cannot be certain of that. His leaving the prayer in the last days can be excused, and prayer does not fall away from a responsible person as long as he is conscious; he prays according to his ability. And assuming he left the prayer without an excuse, the majority opinion is that a person who abandons prayer but does not deny its obligation is not considered a disbeliever, and abandoning it does not invalidate the reward of his good deeds. Deeds are nullified only by disbelief in Allah alone. There is no evidence for the جنازة (funeral bier) stopping due to sending salutations. Charity from the husband's wealth is permissible if the husband's consent is known and there is no indication from him of refusal; customary permission stands in place of actual permission. Your donation for Hajj on his behalf, buying two chairs, supplicating for him, and reciting the Qur'an – all their rewards will reach the deceased, by Allah's will.