Cross linking is a link that is exchanged between two domains. For example domain A gives a link to domain B and domain B also links to domain A. That type of bond between two domains is known as a cross linking.
Guest-posting traffics and ranks depends on your content and the guest post website authority.
If you are writing low-quality guest post on blogs that do not have any domain authority and your purpose is to have backlinks from those domains, then guest posts will not help you.
If you are writing high-quality, authoritative essay on a major website having good domain authority and ranks and is in your industry niche, then guest posts are important for referral traffic as well as ranks.
Flux is shuffling of position of website in the search engine. Flux meaning in SEO -"changing the position of keywords". The changes in existing algorithms of Google ranking parameter are the reason for website flux. When you observe sudden ranking fluctuation for your targeted keywords, that means something is going on at the back end of search engine and that is known as Flux.