Student or Learner
The abundance of trees and shrubs make/makes the trudge into the forest difficult.
I think it should be 'makes' because the subject is 'abundance'. Am I correct?
You are correct for exactly the reason you suggest.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.