Aidylad,
I would think that since most sites or themes offer the "read more" tag for just what you are talking about, it wouldn't be considered as duplicate content. However, I just read an article about article or post summaries or teasers and it suggested to not use the actual content (opening few lines or paragraph), but to be more creative with your hook. the preview should entice the reader and should make the reader want to read your article, so I would consider more of a summary or what is in it for them kind of thing. Just my thoughts

No it will not, Google's algo is smart enough to figure out that it's just a preview/summary.

No it would not. But make sure you are posting small portion of the article on a page and whole article one another page. I never worried about this and never found any problem with this on my 39 websites.

I have done exactly that and did not see any adverse problems at all. I say it's fine also.

duplicate content is the same article being published many times in the same website.

There is a chance of that you can choose some sentence that isn't in that content instead.

No, absolutely not. Google would love such content because if you think of it, this is one of the "ideal" ways to post your content anyway.

I would say, go ahead.