Slander, by definition, involves a false statement. So no, he isn't slandering you.
"if you're going to harp on me for cheating, don't. I've already heard it all and I'm not interested in hearing it again."
The cheating is the reason this is happening. So it is going to be hard to give advice that doesn't bring up the cheating. In order to appreciate the best way to address this you need to be able to fully acknowledge what you did and how terrible it was for your relationship.
Expecting someone to forgive you for cheating and for cheating to not dramatically change the relationship forever is a problem. Expecting someone to forgive you within a week, even more so. Expecting someone not to share their hurt feelings with others and to shoulder all the pain on their own is also unreasonable. He has not forgiven you yet and that is actually pretty reasonable of him. Almost no one who has a healthy sense of self-esteem could completely get over a partner lying to them in this way over the course of a week barring exigent circumstances which you don't mention. His way of dealing with it/ coping right now is to make these remarks and to confide in others for support. He is hurting because of you and it is pretty damn human for a part of him to want you to hurt as well/ be with him in the hurt. You may have been taught to keep your dirty laundry hidden but this isn't his "dirty laundry." He has nothing to be ashamed of. Someone he loves and cares about abused and betrayed him in one of the most hurtful ways possible. You're saying that he is supposed to keep it a secret in order to protect that person? That's not fair to him. You're upset because you think his choice to tell others is the reason you are feeling hurt. The reality is he isn't the one making your friends hate you, if they even do, he is telling the truth about your actions and your actions are causing people to dislike and distrust you. This is all a reasonable consequence of the fact that you chose to cheat. It may not feel fair, but cheating isn't fair either. He isn't treating you with love, compassion and kindness, because he isn't really ready to forgive you.
However, that doesn't mean that you need to put up with it or that it is healthy for your relationship. If his way of getting over your actions is not working for you, you can absolutely chose to leave and that's a fair thing to do. His way of getting over it may actually be unhealthy for you and the relationship. The problem is, you already made your relationship unhealthy by your actions and his way of coping with it is perpetuating that. You two may have agreed not to throw away your relationship but the truth is you already did, by cheating. The question is whether you both are committed to trying to mend it and are in a healthy enough place to actually do so. It sounds like he is still very hurt, and that's to be expected. It's possible that as much as he wants to mend things with you he simply is too hurt to actually have the capacity to do so right now/ isn't healthy enough to do so because of the impact of your choices.
You aren't ready to be married, clearly, and you're going to need to either work on this together over time (which is going to require a great deal of work on both of your parts) or end things. One option may be to find someone he can talk to who isn't a friend and won't treat you any differently, for example a couples counselor of therapist. Put the wedding on hold. Use your own money, not his, to pay for a therapist or couples counselor (if you have enough to get married you have enough to do this first). Visit places like the r/survivinginfidelity and work on fully appreciating that nothing is ever going to magically fix your relationship now and that your actions have forever altered trust between you two. Give it time to mend. If he can't ever forgive you through his actions than the best option is to leave.