Can I cancel a pending I-485 for my dependent spouse while it is still pending?

Good question. This is one that you should probably consult with the attorney who handled the I-140 and adjustment of status case. However, I think that it is probably better to deal with this issue later in the process. From your post it sounds like although the marriage is in trouble you are still legally married. Your wife won't obtain any immigration benefits through your AOS petition until it is adjudicated and approved. I think it would probably just create problems and confusion if you requested that the USCIS withdraw this portion of your petition at this point. I think it might make more sense to deal with this at the adjustment of status interview. To be sure, I think the best course of action would be for you to consult with the attorney that handled your case or retain another attorney to counsel you on this matter. Its probably something that could be dealt with in a simple phone call or email to your attorney.