Do Buddhas and Bodhisattvas care and have compassion for all sentient beings equally.
Perhaps in practice the type of relationship varies -- see for example AN 4.111.
What does "the one taste of the dharmadhatu" mean except that?
I don't know about Mahayana, I think that from the Pali suttas "one taste" might be a reference to this (description of the dhamma-vinaya):
The ocean has just one taste, the taste of salt.
In the same way, this teaching and training has one taste, the taste of freedom.
And, wouldn't that mean that a Buddha could in principle approve of hells
Imagine if someone who you love is in hell, I'd suppose it's difficult to approve of that.
Approving of someone's suffering sounds to me to be the very opposite of mudita (which is one of the four brahmaviharas).
like I believe it is in Christianity, to assure the good and beneficent that their virtue is loved (don't quote me on that)
There are different Christian sects with different doctrines, I heard one of them teach that hell is a self-imposed consequence of choosing to distance or to separate oneself from the love of God.
I imagine Buddhist doctrine is somewhat similar, i.e. that hell is a natural consequence of unskilful or unvirtuous actions and desires. Perhaps the definition of "skilful" is "eradicates suffering".
But the idea of Buddhas approving of suffering just sounds like anathema to me -- instead they would only approve of the skilful/noble ways that lead to non-suffering.
The closest I know of to the idea of suffering being good is this:
And "wrathful" deities (from the Tibetan tradition):