1) Apart from generally showing compassion and benevolence towards all
living beings, in what ways do accomplished Buddhism practitioners
work for the liberation of living being as diverse as, say, ants, pine
trees, salmon and e-coli bacteria? What do the scriptures say, and
what do the more recent writings say on this matter?
Even in the traditional Mahayana systems, not all living things are sentient, (else why would the traditional formulation even include the word?) True, some versions of Mahayana Buddhism incorporate "nature", including trees and rocks. This, but I'm looking for a better reference about the status of trees and rocks.
2) How do accomplished Buddhism practitioners liberate from dukkha the
beings of non-earthly nature of other lokas, such as angry deities and
hungry ghosts? What do the scriptures say, and what does recent
Kṣitigarbha is an Bodhisattva (portrayed as an ordinary looking monk) who rescues people in the hells. Depending on the flavor of Mahayana Buddhism, this is either meant literally, or it is a story about what behavior that a Bodhisattva practicioner should emulate. (Scholar Jan Nattier in "A Few Good Men", where she notes, "After paying homage to the Tathagata at the entrance to the vihara, the bodhisattva is told to remind himself that "I, too, should become one who is worthy of this kind of worship""-- i.e. capital-B Bodhisattva's like Ksitigarbha are to be copied, not so much adored & petitioned for help. (This isn't true for all of subsequent Mahayana!)
His greatest compassionate Vow being: "If I do not go to the hell to
help the suffering beings there, who else will go? ... if the hells
are not empty I will not become a Buddha. Only when all living beings
have been saved, will I attain Bodhi."
The story of Ullambana is about merit a son whose mother has become a hungry ghosts. He holds vegetarian feasts for monks, and then some percent of the merit accrues to his mother so that she can exit that realm.
I think Buddhists went through periods of reification of metaphors and de-reifications of metaphors. When sutras swing towards talking about sunyata/emptiness and the way our mind shapes our experience, then the heavens and hells are in our heads, metaphors for states of mind. And then next chapter they are reified again and people are taking the consequences of these stories very seriously. If there really are hells, it is our responsibility as Bodhisattvas to do something about it.
Interestingly, often the "doing something about it" was deferred to the next life time. Ancient Buddhists were called upon to care for the sick, sometimes called on to give to the poor, particularly beggars, but the bulk of the good deeds done would be done in the next life. In modern times, we're starting to see Humanistic Buddhism and Engaged Buddhism, so while I don't know how effective it is, some Buddhists have their heart in the right place when it comes to dealing with people who are in the metaphorical hells at the moment.