Primary and secondary keywords. Help me out!

Dear warriors!

I have two questions about keywords.
I've searched forum and web for a topic like this, but all the time I'm not getting direct answers.

I found great nichee that suits my knowledge, choose around 40 keywords with low competition and decent monthly searches and alredy wrote bunch of articles.

While writing articles I realized that a lot of my keywords are very similiar to each other. I could say those articles are answering exactly same question. This topic is my bread and butter so I could easly create 40 diffrent articles but I'm just not sure does it really worth it.

Let's assume in my keyword list I have keywords:
green bicycles 500 searches
green bicycle 300 searches
bean bicycle 100 searches
vert bicycle 100 searches

My first question is about plurals.
Should I create sepparate articles for green bicycle and green bicycles?

Second question is about more then one keyword for each article.
As far as I understand each article should have Primary keyword - one that is everywhere, and few secondary keywords - which are mentioned one or two times.
So if in my keyword list there are keywords which are very similiar I should combine them into one article? Or write 3 diffrent articles with main keywords "green bicycle", "bean bicycle" and "vert bicycle"?

Thanks in advance,
Best regards
TS.