Thanos didn't lift Mjolnir, did he? I'm not recalling him doing that. The only ones in the MCU that lifted Mjolnir are Thor, Vision, and Cap on screen (and you can make an argument for Odin).
I believe it was implied that, worthy or not, Odin played a role in the enchantment being part of it anyway, and it's not clear whether the enchantment itself was applied at the time he speaks of it when he casts Thor out in the first Thor movie, but it seems that way.
As mentioned above, I think Hela's situation was (1) her power supersedes the enchantment, so it's not a question of her being "worthy" and (2) she had a relationship with the hammer before Thor was even born, as shown in the painting she uncovers in Ragnarok, which appears to show her with it (which also seems to support that the enchantment was added later). But in Ragnarok, she doesn't "weild" it--she merely stops it and destroys it, which, going back to point #1 seems to show that she was simply more powerful than the enchantment.
And as Adami said, there was no such enchantment placed on Stormbreaker.