From what I hear from everyone, it sounds like the fault goes around to everyone there.
The fourth ref should have immediately noticed that there was a substitution in without someone coming out.
The Wando coach should have kept track of her players coming on and off the field and not have waited to see if Lexington would do something before doing so herself.
The Lexington coach should have gotten up immediately and gone to the fourth referee and told them, calmly, that there are 12 opposing players on the field. (The impression I get, correct me if I'm wrong, is that the Lex coach stood at the sideline and just complained loudly, hoping the refs would notice).
The Wando players should have noticed they had an extra player and said something, especially the captains and even moreso the player that came in as it is usually required that they wait for their man to come off or be heading off before they run on.
The Lexington players should have noticed and immediately had their captain tell the center ref, calmly, "Hey, I think they have 12 players, when you get a chance, please count."
The center ref should have been paying attention at the substitutions to make sure that subs did not enter the field until the exiting player was identified and making their way from the field or off of the field entirely.
The sideline and center refs should have seen or heard the crowd and taken a quick moment to count, even if it was when the ball went out of bounds.
Unfortunately for everyone, it's hard to pay specific attention in the midst of an adrenaline filled game and even moreso at the end of the season's biggest game, whether you're winning or losing, and it's often hard to hear the crowd when you're focused on the game. Logically, to me, the answer would have been to add the minutes on the clock that were lost with 12 up (it sounds like it happened at the final substitution) and play them fairly eleven aside; but then I was not there and did not see what happened and how exactly the added player affected the game.
This is an awful cap to a great season for two teams and is something that will taint the championship for both the seniors that won and those that lost.
I too have plenty of experience losing games due to one crucial referee decision--or lack of one--however, as I remind the girls I coach now, you had 75 (or whatever number is appropriate) other minutes to put a score up on that board that would have negated the referee's mistake.
Soccer, unfortunately, is a game where, ultimately, all that really matters to win at the end of the game is that you scored more goals than your opponent did. If you can't get it done in the time you have, whether it's 50, 60, 80, or 90 minutes--or even overtime and pks--then you don't win. Knowing that, deep down, always sucked for me because it was hard to accept that we could have made sure that a mistake didn't lose us the game...but I always had to accept that it was true; we had shots we didn't connect, didn't take...defensive mistakes we did make, blocks we didn't get.
I applaud the efforts of both the Wando and Lexington players and all the time, energy, and passion they put into this season. Both were deserving of making it to the title game and I'm sure both sides played their hearts out. All I can say to them is to take heart in that you played 150% and you played with your heart. Whether you won or lost, dwelling on it won't help when (it sounds like, at least) there's nothing left that can be done.
Take pride in yourself and your team for playing hard. May your seniors move on to a memorable college experience with or without soccer, and may your underclassmen be fueled to success next season.