the cancel effect is affecting the room, not that moment in time. . .it's sort of like smashing all traces of everything up to that moment. . .Or trying to.
Like, completely unrelated, another seer is doing a research paper on the victim's grandfather, and tries to look back. . .annoyingly that one room is a pain in the arse to look at for any time previous to Tim's cancel event time point 10 (even 20 years previous, you still need to get past that Jam point.)
The cancel screws up any "past visions" attempts from the moment of spell effect, backward to the beginning of time.
Use that time travel spell to go before the event or the cancel, then use future visions to look at it, and yes, you would be able to look at the murder from that angle without an RR challenge. . .that's a 50th level astrologer spell though. It would probably be more likely that you could just purchase another rank in that list, and make another RR vs RR attempt, and just keep doing it to try to get through. (The recurring plot hook). . . .of course, if the cancel was a 300th level arcane ritual, you may never get through.
Cancel channeling would screw up DT. . .I consider that a feature, not a problem.
As to radius, say it's 20'. . . .if you fail on the RR challenge, you can observe all you want outside that radius, but inside it would just be a sphere of nothing in any attempt until you beat the RR challenge. So the use of the cancel is limited, in two ways, first, it's limited by the level of the effect in the RR challenge, and second, it's limited in range. . .you can't peek into the range from outside, but you could infer all sorts of things by observing outside the cancelled area. Like "Two men enter, one man leaves". . . .and the one who stays, you later find as a dead body. . .that doesn't proove the man who arrived and left is the killer, but it's rather suspicious.