The first question is hardest to answer because their a lot of different ways that an LLM will help in writing a paper. Yes, there will be some people who don't, but over time they will become a minority.
The other questions are easier.
The straightforward answer is that right now, openAI have said that you should acknowledge its use in publication. If you acknowledge a source, then it is not plagiarism. So currently a practice for some journals is you have an author contribution list, where you list the different parts of an article and which author contributed to them. e.g. AB contributed to the design and writing, GM contributed to the writing and analysis etc... One can imagine then you would add a LLM (and its version etc...) to the contribution part to make it clear its involvement. If this became common practice then it would be seen as unethical not to state its involvement.
Language models like ChatGPT have gained popularity in recent years for their ability to generate human-like text. In this context, it is natural to wonder if a tool like ChatGPT could potentially be used as an aid for writing a thesis or scientific paper.
As a non-native English speaker, I experimented with using ChatGPT to write introductions and to edit/rewrite my sentences to make them sound more natural in English. Although the text generated by ChatGPT was well-written, it did not include any real references.
For editing, personally, I found the text produced by ChatGPT to be a superior alternative to my own writing, although I may be overly critical of my own writing style.
Doing this experiments some concern came to my mind and I am sure that I am not alone in this