You can alternate temporary write files with overlapping time frames to reduce the total data written and then discard temp files when they are no longer needed. You still record the entire game but overall you use less storage.
Your possible solution above wouldn't work. Since there is no way to know which mob would deliver the death blow, the only way to possibly do it would be to record every mob's PoV until that mob is killed and depending on the mob, some time afterwards as well. So if there were 260 mobs on a wave, the computer would have to record 260 PoVs. As each mob is killed, the recording PoV video for that mob would be terminated immediately, or depending on the mob, continued for a short while afterwards. After the wave is finished, all of the recorded videos would be discarded.
There is no way to "overlap time frames" to reduce recording. That strategy would increase the stress on the computer dramatically.
Another possible way to record the death cam video would be to only turn on PoV recording for a mob is only when there is an attack by that mob. That would eliminate the travel times for a mob being recorded. Of course, to reduce disk activity, the buffer time for each attack would have to carry over to the next attack or a few seconds afterwards. The problem with that strategy is that each computer would have to record the PoV from certain mobs that weren't attacking that specific player directly. Doing the calculating and switching using that strategy could be more stressful for the computer than simply bruteforce recording all of the mobs PoV.
Since these videos aren't being saved and discarded at the end of the wave, these videos, conceptually, could simply be recorded directly to memory. The problem with that is the recording resolution. If someone's video resolution is 1280x1024, or the death cam is hard coded to use that resolution, then it is entirely possible that if someone has a free GB of RAM and maybe a spare core for processing, this could be done with a minimum of impact. However, if someone is running a higher resolution like I do at 2560x1600, then recording to memory isn't possible.
The easiest way, and least taxing, would be to simply continually loop record the last 10 seconds with a camera above that specific player in memory. It wouldn't be from the mob's PoV, but it would show that person's death and I think that would be cool.