I didn't participate in Source a Thon. I just did my regular wikitree work, by working on certain Suggestions. I also added plenty of sources to profiles, removing the Unsourced template when it was found, so the Source A Thon counts won't include my work, but there are plenty of profiles that now have sources.
In reviewing recent changes to my Watch List yesterday, I found quite a few profiles that had a change made because of the Source a Thon. The last change showed that someone removed the Unsourced template and added a source, BUT the profile already had a source on it. They took the existing source that had been the child listed while searching for the parent, and added an additional source with just the child's name. It is the SAME source, no reason for a new source to be created, but that was done multiple times, removing the Unsourced template. Agreed the Unsourced template should have been previously removed, but I am sure that plenty of people sourced a profile to get credit when there was already a valid source on the profile. That also is 'cheating' the system when there was a source on the profile. Also, the profile needed to be cleaned up because there was no reason to have the 'exact same source' included twice on the profile.
To stop people from doing pre-sourcing, I am not sure how you are going to know that people have done that, other than seeing too many quick updates being done by one person 'because they only have to do a copy / paste' from the pre-sourced document. The same quick updates can happen, though, when a census is found for a family. It is added to the father's profile, probably because it is easier to search for the father, but then it can be quickly pasted into each of the other profiles of the family members. That person didn't do pre-sourcing, but they will have quick update happening, at least for a time.
Giving incentives for most profiles updated does not seem like a good idea because there are always people that will try to get the top numbers and putting quality into the profiles is always better.
Similar things happened during the Clean a Thon when some teams took certain suggestions and did not add sources, as the suggestion stated, and only added headings to empty or nearly empty profiles, which is a very quick copy / paste to be done. Yes, it added headings, but the suggestion also stated to add sourcing which everyone should be doing to all profiles that have no sources.