You can also switch players between lineup and bench if their lineups haven't posted after the first game starts, but doing so turns off auto subs.ah i forgot that you can change the cap, stupid if you ask me. If you lock the roster you should lock the captain. I mean its not hard to figure out who you captained after a few games. 1st of all logic dictates you are capping a favorable matchup, 2nd I can still do the maths and see when your cap played or didn't play to a degree of certainty and whittle my guess down further.
And if it does come down to your scenario (btw the defender is superfluous information all you had to do was dueling captains and lock in the win by matching captains) then its not really fair to the guy who sets his team and goes off to do real life stuff vs someone who is a diehard fantasy player who is glued to their screens. Its tantamount to giving someone 1 free roster change post lock if say a surprise injury occurs or like your scenario he lucked into a guaranteed win by changing his lineup.
And I threw in the defender just to forestall the objection of "how often will you and your opponent have exactly 2 players left, and in common, but captain different guys?" But yeah it was largely superfluous.