You are absolutely correct. The complaint can be amended to change the felony, add a new felony, add victims, etc. The charge can also be amended to "plain old murder" instead of felony murder with the District having to prove intent, malice aforethought, depraved indifference - whatever the statute requires for a charge of murder 1.
Interesting question you raise about arson. I am not sure about arson, because I believe felony murder requires that the victim died as a result of the violent felony. I thought the standard was that the death had to be a forseeable result of the felony. If someone were dead before the fire started I am not sure that meets the standard, but I honestly do not know the answer to that. Trust me, I could be absolutely wrong. Wouldn't be the first time today and it isn't even 8:30 am, LOL.
Please do not think I buy the defense arguments for a second (you can't prove he was in the car, he didn't really get the money because JW took it, the money belonged to some other dude, etc.). Neither did the judge. I was just pointing out where I think defense was going, and why they are so keen on making JW an issue - to cast reasonable doubt on DW's participation in the underlying felony, whatever that felony turns out to be.