M., 20 month old kids are NOT stubborn, they are CURIOUS TO EXPLORE, which is so totally normal and good! :) Please, do not worry about it at all.
why do you need him off the counter?
I would take off/from that counter everything that might be dangerous/not desirable for being touched by the curious boy objects, and place his toys there. See if he continues approaching the counter with the same interest: he might lose all the interest very soon, also :), but even if not, soon he will find a new space to explore, forgetting the 'importance' of that counter. Yet, if you insistently draw his attention to the fact that this is a forbidden territory, the more interested in the space he will be!!! So, it is a hard battle, and I suggest you rather win by being wise, not insistently prohibiting the approach...
Microwave: is it anyhow dangerous? Can you move it higher up where he won't get to it? If not, there are many ways, but please, no fight! You can always invite him to see what is going on in the microwave, so that his curiosity finds the outlet: putting food from the freezer in, let him touch the frozen food, then show what you do, let him stick his head into the microwave while you are right there, then close the door TOGETHER, and after the defrosting is completed, let him touch the food again... but insist that he does not push any buttons and does not open the door WITHOUT you. Say one word (they cannot follow many different instructions as easily, as just one word), for example, the word can be "TOGETHER!" You want to see the microwave, okay, we do it TOGETHER! If he goes there alone, you watch, and say immediately TOGETHER, and go with him. then, together move away from this "treasured chest". Of course, he needs to know the word NO, but I always tried to avoid unnecessary prohibitions, providing my kids as much space for exploring as I ever could. I don't mean it badly, but with little ones, some things work like when you train a dog: to develop certain reactions, especially, if the need for prohibition is about preventing dangerous things from happening. the clue is, then: the ordered prohibition comes always with the same word, same facial expression, same insistence, NEVER changing the outcome - meaning, never give up and allow something which is usually prohibited, so that he develops a pattern in his mind, that this is ALWAYS the LAW!
when unwanted behavior started, I tried to prevent it, and after saying my NO word, I hugged them and cuddled, and distracted their attention on something more interesting, and it works much better than fights, really... you are a Friend, right, not a constable, and a safe HAPPY environment works wonders for the entire family and each representative :)
Kids grow too fast, mine are 25,23,17, and I really miss that time of their being little Great explorers, which at that age and on, for a long time, they ARE!
I wish you all to stay very happy, and enJOY the discoveringS
of the environment, and inner movements of the feelings and thoughts!!! :)