You obviously could however Google doesn't use the meta tags as far as I am aware and you may get penalized for spamming of some kind.
I have a slight different opinion. Google surely takes meta description while indexing while it doesn't use meta keywords. This can be seen while we search in Google for any keyword the Title , URL as well as description or on page contents are displayed in the results. I think it is better to use unique Title and meta description for each page. Meta keywords you might just add relevant keywords 4-5 if you want to use them.
Google do count meta description on the source page. though you can use same content as meta description but it will be better from SEO point of view as well as keyword targeting that you use the different content for both of them
Yes, now meta keywords are not so important and not considered for Google Rankings. There is a possibility that the algorithm might change at any time so better use 3-4 meta keywords.
Probably meta tags aren't as important as they used to be. Still they influence SERPs. Of course you should make your meta tags unique and not just copy and paste your website content. Meta tag should include your keywords but it should be a unique message.
meta tags should be related to the content of the page so i also think that adjusting or using unique meta tags on every page.
meta tags are only for short descriptions and not for a 1000 words descritptions..... as such there is no limit but as an standard the word limit for meta descitption is 250 words max......
I wouldn't copy the meta description into the page's content. I have heard that things like this can lead to an over-optimization penalty. At least vary the text slightly if you are going to do this.