Is the r deliberately inaudible?
No, it isn't.
Can the sentence be just: "Ma casarnap," without the "van"?
Indeed, it seems to me that it SHOULD be without the 'van'...but I'm not a native speaker so I can't be 100% certain. Where is vvsey when we need them???
No, actually it's necessary or at least it will sound lazy without "van". The dummy "it" expresses a condition of the environment around you, time in this case. In these scenarios, "van" is necessary.