Dear Tamara, I am sure that they will get the letter and read it. Marcus died only 4 days ago, so it is unlikely that they have already packed all his things and gave off his place. Don't worry, they will get your letter. In Germany and also in Austria, it is customs that you give a so called "forwarding order" to the post to have any letters to be sent to another address when someone moves or dies. So, even when they do not have his place anymore (which is unlikely), the letters addressed to him will find their way to the parents. And don't worry about what you wrote. It is normal that it is a lot about you and I think this is good. So his parents get to know you a bit better and know a bit better how your relationship to Marcus was. Maybe they do not know much about it. I think that they do not know how to contact you or are not very familiar with a computer. Your letter will help.
As I said, if you want me to help with a translation to German, here I am. You can send the letter via personal conversation if you want.
A big hug.