This event needs some reworking due to the circumstances. I'd suggest that you cut the prize pool down to be able to make multiple events rather than a one off event and go for 6v6. Use the 6v6 to promote activity, it will be easier for players without a team to find a full team to take part. The problem is finding a 10 man team for the majority of players, very often people queue up in 5 man premades so finding an additional player to take part alongside their usual 5 man premade roster shouldn't be too difficult. I'd guesstimate that you can easily find 4-6 teams this way.
Don't let the hype of this event go to waste, make changes, promote activity and aim for 10v10s at some point in the foreseeable future.