As for the voice acting being different, well I'd have to check that out with the subbed version, see if it's something specific to the original vs whoever is doing the dub. How a character comes off can be heavily affected by the dub, even if the script is the same. With Hachiman being a cynic, it would be easy to skirt the line from resigned cynic to haughty asshole pretty easily just by inflecting the lines differently.
No, it's much beyond simple line readings, it is part of the animation. Two examples: 1) Hachiman geeks out over seeing a vending machine styled in the form of his favored canned coffee brand. He's almost a starry-eyed teenager swooning over meeting a pop idol in person. 2) While in class, he gets a message that the results of the entrance exams have been posted. He immediately jumps from his chair and breathlessly runs to view the postings to confirm whether his sister passed or not. While his affection for his sister is cloaked by cynical jibes (with Komachi returning as much as she takes), the energy expenditure in these scenes well exceed anything he's shown previously. As I say, maybe the LN showed more of this side of him earlier and it just wasn't included in the S1 and S2 adaptations, making it seem inconsistent when they
do include some of it.
Comparatively, a bit later, he also runs to help Yukinoshita, but that was a time-sensitive matter and he had some distance to cover. Maybe there's a cultural disconnect and I just don't understand the urgency in knowing the results of a high school entrance exam.
filled with the annoying para-language (gasps, hums, etc) anime trope
Is this a dub-specific thing? Maybe that's due to having to pad things out to allow for differences in the pacing of the language when it's translated. If it is anime-specific that's probably because anime is the largest segment of overseas dubbed content people are familiar with.
As I understand it, it is a Japanese thing, but it gets ported into dubs where it is extremely out of place. English speakers have their own sub-verbal vocalizations, of course, but to simply copy them 1:1 from the original language just doesn't work. It sounds unnatural. It's not due to padding; if anything, English translations are often feel rushed to me, presumably because it is trying to translate a concept or phrase that is easily or quickly communicated but has no simple equivalent in English. I certainly
hope they're not adding extra sounds that didn't exist in the original just to pad a scene, but then there is other behavior in dub translations that don't make sense to me either, so what do I know.