Can v2 still be done in English by using the do support

no clue what V2 is, but “At night, do I eat sweets?” can only be a question. Also, it’s a question that makes no sense unless you are in the TV show severance or something

That's either a question, or poetic licence.

I've been speaking English a long time (i.e. my entire life), and I have no idea what V2 is.

Actually that kind of phrasing wouldn't be out of place in poetry. But in everyday speech, never.

English is V2 most of the time but it’s not as strict as in other Germanic languages. Your second example turns “I eat sweets” into a question. This is a context in which V2 doesn’t work and it has to be put later in the sentence.

what the fuck

"Do" before the subject is **always** a question.  "Do" after the subject always has to come before the emphasized verb. So your sentence sounds like a question.

The only time I can imagine you could use the phrase "At night do I eat sweets" is as an emphatic, casual sentence to express that you eat a *lot* of sweets at night and would be emphasised as "At night, do *I* eat sweets!" Sometimes you'd hear this sort of use in a call and response like: Q: "Do you eat a lot of sweets?" A: "*Do I*?!" Where you are essentially asking a rhetorical question where the emphasis is telling the other person that the answer to that is very much yes.

Yes, it's grammatical. As usual, lots of natives claiming something is "wrong" just because it's highly uncommon. It also need NOT be an interrogative; "at night do I eat sweets" is perfectly acceptable to express "I eat sweets at night." A usage note should clarify that this reads as arched syntax to most speakers, and would most often be used for effect rather than for straightforward communication. u/culdusaq provides the useful further note that "Only.... do I/does he/etc." is the most common collocation; I expect the vast majority of native usage comes on this form.

Unless phrased as a question about eating sweets, no. "Do I..." is the interrogative inversion of "I do..."

No. You can you "do I" as a way to answer to questions with "do you" Eg "Do you like chocolate cake" "Boy, do I ever!”

No, it doesn't work, unless you make it restrictive: *Only* at night do I sweets. With that said, people would rarely actually say that.

No, English doesn't work like that. "Do" support in English is for questions, negations, and emphasis, not for making sentences verb-second like in some other languages. "At night do I eat sweets" is not standard English for simply saying "At night, I eat sweets." It sounds very strange and unnatural. English word order is subject-verb-object, and while you can move things around for emphasis, you can't just insert "do" to create a different basic sentence structure. Your example sentence using "do" is not correct for typical English.