This is getting interesting, for which I thank you.
Now... unfortunately, I disagree with almost the entire content of your post. However, I am not saying that you wrong, just that I have a highly different opinion.
Paragraph 1: The second to last phrase, I assume you meant to say person B, because otherwise the phrase makes zero sense. Also, I said "better", not "good". If I am allowed to have only one snapshot worth of data, I want to have one taken at a time, where the population online is likely to be high(est).
Consider the following two examples:
1) You get one snapshot, size 125 characters, timepoint unknown. You can't do much with this. Now, you are given a time, 20:00. Now you can tell that the population is likely to be low OR there is something else unusual going on.
2) You get one snapshot, size 2913 characters, timepoint unknown. Without any extra information you know that this snapshot was NOT taken during early morning and the faction is very unlikely to have a very low population.
In other words, I said better, because in my opinion, if the number samples is very low, the value of size of those samples increases. In short, having one small snapshot is worth less than having one large snapshot.
Now to the longer part:
Quality: I have to disagree, at least according to your later example of my example. The quality is increased if the latter snapshot was taken at the same time on some other day. That information, however, was intentionally not in my example. So the quality of the data can not be determined accurately.
Quantity: I disagree, time is not the relevant factor, but for a different reason: The relevant factor is the number of characters in the examples. Again, an unknown factor, because we don't know if the two snapshots came from the same or different factions. For example person B could have very heavy overlap if the snapshots came from the same faction and zero overlap if they came from two different one. Person C would have the least amount of overlap even if all four had chosen to snapshot the same faction twice.
Accuracy: Question mark sure... but the reason is different again... If all eight snapshots came from different factions, we can't interpret much... but if all four people censused the same faction, we actually start to see things.
Next going to your example...
You ran into a brick wall, head first.

Your example already assumes something you did not consider or chose to ignore. Running two censuses at 2 AM will find a very small number of characters. Both the scan at 5 PM on Saturday and the one at 12 on Sunday will most likely find (a lot) more people online.
The two at 2 AM have high probability of overlap, so the data quantity will not increase much from the first to the second. The scan from 5 PM will probably be bigger than the one from noon Sunday... even so, in my opinion the following is valid: The superior information is provided by the person who saw the most characters twice... and that person is more likely to be person B. For person A to produce superior information, person A would need to be on a server where almost noone who plays on Saturdays at 5 PM, plays on Sundas at noon.
I admit that situation might occur, but in my experience, it is unlikely.
Now on to the last paragraph...
Ummm... I am sorry, but just no. If person A saw 3000 different characters once, he provided less information with equal accuracy/quality. And if he saw the same person 3000 times, well, the data is super accurate/of great quality... BUT... it is basically useful to only one person. Person B can't win the quality/accuracy race, but his data is useful to more than 3000 people, no matter what person A did.
You see, in my opinion, the best census data provider is the one with the best information to censuses ran ratio (as long as the number of censuses run > 1). Person A runs 10 censuses at 100 units of new or updated info and scores a total of 1000 new & updated, person B runs 100 censuses getting 10 units of new & updated for an equal total of 1000, while person C runs 1000 censuses and gets 1 new or updated each time for a total of 1000 as well.
Person A did the best job with a ratio of 100 to 1, person B managed a 10 to 1... and person C really should have done something differently with his 1 to 1 ratio.
Now the question remains... if person D runs 1 census and scores a total 1000 as well... is the data worthless? In my opinion, no... On its own it is of low quality and accuracy, but it still benefits 1000 people slightly.
This conclusion might also interest you... I may hold both top 10 first spots, both overall and montly and there probably aren't very many people here who do 1200+ censuses per month... but there is a very large probability (approximately 99,998% if I calculated correctly ;D ) that the best censuser is someone entirely else, most likely someone we can't see on the lists at all.
Why did I end to this conclusion? Simple. Given the same amount of time, someone else could have run more censuses AND collected more (new & updated) data.
This is turning into another wall of text, so I am going to take a break... and reread the thing a bit later, in order to try to spot my possible goofs. :/