There is no specific rules but avoid duplicate content as much possible.
Normally, It is not bad if you use an article in 5 to 6 websites. Make sure that articles published in your website should not be published elsewhere. You can promote the link of your website's article but not the entire article with content.
Yeah, Guest posts increase your backlink volume. But be careful for guest posting, it may harmful if you publish guest post in spammy blogs. I have reaped the benefit of Guest posting for one of my website FreePriceCompare.com
Have u ever read the definition of bounce rate given by the Google itself in Google Analytics?
As per the Google, "Bounce Rate is the percentage of single-page visits (i.e. visits in which the person left your site from the entrance page without interacting with the page)."
Lower bounce rate or higher bounce rate has no concern with your ranking but yes, how much time visitor spends on your site or page is matters. If visitor spends adequate time on a page and left the site then it won't consider as bad thing for search engine ranking. Yes, Average session duration and Page/session are two most important factors you should keep in mind while analysing your website performance..
For FreePriceCompare website, we applied all standard practice for speeding up the website like image compress, simplify the scripts and other things. Server response plays an important role in order to speed up the website so please make sure that you are on the high-speed server.