Yes keep it simple. If you don't have good proof of abuse just go with the divorce waiver.
If he is willing to send you a letter then it may help. If he only wants to send it directly to USCIS then I would not trust that he doesn't try to sabotage you in some way. If he is anything like my ex he will keep trying to mess with you every chance he gets. I would not trust him to send anything to them.
Dig up everything and anything you can think of for good faith marriage. If you take a look at the ROC forum you can see what some others are sending. Heck we send everything but the kitchen sink.
I am sure there are rumors but don't believe it unless USCIS officially releases a statement. Historically they moved away from conducting I-751 interviews. If you go to page 19 of this document https://www.dhs.gov/sites/default/files/publications/cisomb-conditional-residence-recommendation-final-02282013_1.pdf ; the bar plot shows the distribution of received/approved/denied/ and interviewed cases over years. In 2008 the approved/transferred ratio almost looks like 2:1; that's crazy. In 2009 that rate dropped further, and in 2013 it looks like the ratio was 8:1 ratio that suggests only 10-12% of the cases get interviewed.
I-751 is a burden petition; pretty much everybody gets approved. official average time an officer spends on a I-751 case is 0.99 hours; so why increase the workload of field offices with something that would almost always get approved? That being said given the backlog in both service centers I wouldn't be surprised if they implement some radical changes such as give conditional GC to couples that only exhibit higher fraud risk at the AOS status and reduce the number of people who file ROC; right now that only depends on the length of the marriage. If you are married for 2 years by the time you get your GC; you skip the conditional residence.
