Can "almost half" mean a little above half? The lesson states this could mean 49% or 47%, but is it possible for it to mean 51%? Or would that be incorrect?

Struggling with the language in this question.

Most large nurseries sell raspberry plants primarily to commercial raspberry growers and sell only plants that are guaranteed to be disease-free.

I understood the phrasing sell only plants... to mean the only kind of plant that is sold is disease-free. I am confused at how the most comes into effect this clause. I am taking the explanation at its word when it says that this is not the case, but I am struggling to actually see how that is true.

I understand this question after the breakdown and diagramming, but I'm not sure that I would be able to reach the answer while taking the test online with the limited highlighter/ underline tools (honestly this is a concern on lots of questions). I am a very visual person and I wish the LSAT was still widely offered on paper :( Do people have tips on completing complicated questions quickly without drawing/writing on the question?