Another suggestion that may be worthwhile is expanding the 1v1 format and turn it into an investigator team of players against an even amount of keeper players assuming the role as the keeper.
So let's say you have 3v3. What would transpire is the three players would each have an investigator to control, and is as normal rules for the investigator team. The Keeper players however must decide who goes first as Keeper for the round (whether this may be utilized by group decision or die roll). This must be determined after set-up, as that keeper player may wish to play mythos and trauma before the keeper reaches his first turn. Likewise, the appointed player assumes control of the Keeper for the round. Next round one of the other two remaining keeper players would decide who goes next. The round thereafter the last player assumes the roll. This assignment from now on stays in that order (use markers to represent this). The Keepers may each give one another advice but the active keeper makes all final decisions for his round of play. They also each are knowledged to what the objective is and must be careful not to alert the investigator players what the objectives or next clues are. Each team may use a silent form of communication, whether it be texting or paper (or even whispering to one another). Players may not communicate though when someone is attempting a puzzle.
In a 2v2 game, the investigator team is always given the choice whether they will each play with one investigator or two investigators each. One player may not have 1 while the other has 2; the format is strictly 1:1 or 2:2 investigators.
Also, what time frame are you looking at? Is your assemblage lasting 8 hours, 12 hours, 24 hours each day!? Do your points carry throughout the weekend? How do you deal with absences? Will there be intermissions? More and more questions, I have…
If you have large time-frames with a small assembly, I recommend best of 3 games. The winning team gains point while the losing team assumes the opposite role they just played and chooses the next map (if you will allow players to do so). I recommend random A B C and # format when Keeper does set-up. In case of tie games:
if points are currently tied, the next game with a victor wins at their table
if one team has one point more than the other side, that team wins at their table
At the start, take your total amount of contestants and divide by the board games you have available. For instance, if you manage 26 contestants attending, and have 6 cores, you will end up with five 2v2 tables and one 3v3 table. Alternatively, if you have 23 contestants and 3 cores, you will end up with two 4v4 tables and one 3v4 table.
There is the chance you will have from your total an odd man (woman). This final player is teamed with the first table and becomes a "n" versus "n+1". I don't see this negatively formatting your game, as the Keeper assignment of respective rounds should balance out. Always keep your tables low on the amount of players when able, as dividing the Keeper's round to being only the active keeper who makes decisions will work better with low assemblage.
Draw players out of a hat to determine table arrangement and teams. For example, Eddie is the first name drawn out of the hat, and is assigned to table 1. Becky is the second name, and goes to table 2, so on and so forth. When a person is seated at each table, cycle back to table 1 and allocate a new player there, and then to the 2nd, and so on and so forth. Each odd draw to a table is a Red team member while each even draw to a table is a Blue member. When all players are seated, the red team decides if they want to choose the map or the player role. The blue team then chooses the option they are left with. The first Red member always gets final say, as does the first Blue member get final say when determining these choices. For example, Eddie counsels with the teammates that they choose "Role" and they will be Keeper. Blue's Sam counsels with his teammates who are left with the choise of map, and they debate between "Green-Eyed Boy" or "Blood Ties". Because Sam was the first Blue member, he addresses Red team they will set up the "Green-Eyed Boy", as Sam believes a "Blood Ties" Keeper would result in the investigators being slaughtered mercilessly.
When a team has won, they are reassigned back into the hat with the other victors and drawn again to create new tables and teams. Rinse and repeat until you have a final set of players. Your final table match should be 1v1. If your odd man survived every team he was put on, play a round of Call of the Wild's "A Matter of Trust" (assuming you have it). The first player to be killed off is eliminated from the tournament. The scenario proceeds until it is over. If there is a sole victor, he chooses which map and which side to start with. If there is two victors or no victor, the last to have died chooses either map or role, and the second to last to have died makes a choice with what is left. If both or all three investigators die at once during this match (or all live), the match is restarted with the final participants of the last game round till there is a clear factor in deciding the next match.
If you do not have Call of the Wild yet you do have a 2v1 dilemma for your finale, draw a name for the Red member. The other two instantly become Blue team. If Red ever loses a match, he is eliminated and coin flip determines which blue member is now red. If Blue team ever loses a match, the Red member gains a match point and a coin is flipped, heads representing one blue member while tails the other. The side revealed is the blue member who then defects into a red member. Then a second match occurs. If red wins, blue is eliminated and another coin flip produces who stays as red and who becomes blue. If blue wins, then the blue member who defected to red is eliminated and blue gains their point. Then continue as normal until one victor remains.
Again, so much of this depends on your board game to player ratio. If you have lots of Mansions copies available, you are pretty well free to opt for however way you want to set your tournament up. If you are limited on your board copy supply, I recommend 3v3, and 4v4 if you got way more than you expected.