I think the last option is by far the best one.
The top rankers for "earn cryptocurrency" are big sites so it will be hard (pratically impossible?) to compete with those with just a single article, no matter how long you leave it.
The second option is not fair at all since there's a chance that many of the articles won't have used the keyword phrases/keywords. Not an even playing field.
It also depends where you're checking the results from. Yours will be different from mine. UK will be different from US etc.
Thanks Gillian
I think I prefer the final option for the reasons you have stated but want to see what the sponsors think - they may think that the prizes are not to be issued as Google shows no results!
Good point with the UK/US/etc locations - just another reason why this contest was going to be a brain ache!