How to prevent selfish mining?
I do not really understand any of given solutions. Can someone please explain in plain English

I also find this stuff difficult to understand however I'm a little skeptical of the idea that complex issues can always be simply explained. There is a reason why people spend decades studying a subject in order to understand something. Selfish mining isn't one one of these, it isn't quantum mechanics or some esoteric branch of mathematics, however it probably isn't simple.
I find this website works best when you ask one, specific question rather than bundling a bunch of questions together. So far as I can tell, this isn't a place where you can routinely expect volunteers to write substantially long multi-page essays.
A google search finds Majority is not Enough: Bitcoin Mining is Vulnerable; Ittay Eyal and Emin Gun Sirer; Department of Computer Science, Cornell University which, looking at the first few pages, seems to be readable. I would start there and ask here about any single specific points that I needed clarification for.
Cornell is a reputable institution and a peer-reviewed paper with substantial citations ought to be a reasonable starting point.