I think a key property of a deus ex machina is the abruptness of the occurrence. In vulgar Latin...an asspull. A lot of these examples had years of set-up or serve a narrative purpose.
- Yeah finding the locket was a bit too serendipitous but Kreacher has been shown before to obey Harry _contemptuously_. When asked to report on Draco's activities in HBP, he reports the most mundane things, making his report essentially useless without breaking his obligation. He only finally cooperated when Harry looked at him as a creature with feelings, not just a servant. Ties back to when he asked Dobby to sit down in CoS, which sends the elf wailing at the decency.
- Polyjuice has been established since CoS, along with the cost of using/making it and its limits. Hardly sudden and hardly an unfair advantage.
- Did a Voldyvision ever lead Harry directly to a Horcrux? I don't remember so. The closest I recall is Harry willingly slipping into this trance to verify Voldemort's anger at discovering the loss of the locket---not a huge advantage and a reasonable tactical move. The diadem, Harry had to rack his brains for that, even empathize with a ghost.
- Voldemort couldn't Avada Kedavra Snape because he believes Snape is the Elder Wand's master.
- Snape lied to Voldemort to the very end, his legillimency is not infallible. Sending Narcissa Malfoy is a bit of good luck, I'll grant, though it also emphasizes how one of Voldemort's weaknesses is his inability or plain refusal to read people. He failed to see that Snape loved Lily so, which caused him to turn for the good, and he failed to see how much Narcissa cared for her son---he didn't consider she'd have a reason to lie to him.
- Ron "speaking" Parseltongue as a plot turn feels a bit rushed and unsatisfying indeed but is not at all miraculous. I can say "Spasibo" to thank my Russian coworkers even if I'm not Russian and hardly has training in Russian.