Question: If Amitabha Buddha (Amituofo) lives eternally, would this not contradict the Dharma seal of all conditioned phenomena being impermanent?
Answer: The answer is in the question already. Impermanence as a seal of the Dharma applies only to all conditioned dharmas (phenomena of mind and matter) of defiled worlds. Buddhas have attained the unconditioned state of Buddhahood, which is permanent (timeless; eternal). Thus can Pure Lands be eternal too, due to Buddhas’ immeasurable meritorious virtues powering them for guiding beings to Buddhahood. Here is a related quote:
The Land of Ultimate Bliss [Amituofo’s Pure Land] is the unconditioned realm of [or ‘for’] Nirvana,
difficult to be born in by following [changing] conditions and cultivating mixed forms of goodness.
This is why the Tathagata chose the essential method (Dharma),
of teaching us to be mindful of Amituofo, by focusing and repeatedly focusing [upon his name sincerely, so as to connect to him via the Three Provisions].
– Pure Land Tradition’s 2nd Patriarch Great Master Shandao
(Praise of Dharma Matters)
If Buddhahood is not permanent, and is instead capable of backsliding, it would be pointless to practise to realise it. The first of the Four Virtues (四德：常乐我净) of Buddhahood is permanence. The Dharma seals realised are just stepping stones to realise these Four Virtues.
There is also a common misunderstanding that when Buddhas enter parinirvana (relinquish their physical forms), they will become absolute nothing. This is not true as the final spiritual goal is not to commit spiritual suicide, what more when there are immeasurable more beings to be delivered from suffering. Buddhas have simply transcended samsaric cycles of birth and death. Buddhas do not ‘die’ or become nothing, as they have endless compassion for all beings to be helped. They simply manifest elsewhere to guide more beings.
Thus did Sakyamuni Buddha reveal in the Lotus Sutra, that Buddhas have eternal lives, that they manifest parinirvana only to urge beings to treasure their limited lives, to practise the Dharma while they can. In the Brahma Net Sutra (not Sutta), Sakyamuni Buddha also revealed that he had re-manifested walking the path to Buddhahood 8,000 times already. Such is the truly great and perfect compassion of Buddhas!
The Buddha Is Still Around
Is The Ultimate Goal To Become Nothing?
The Middle Path Between Nihilism & Eternalism
Is The Thus Come One Gone Forever?