Playing a lot of C&C PBEM and looking back over log files during the period when I was returning turns promptly (1-3 turns per day), many games were completely within a week, almost all with 2 weeks. Typically 30 log files created (range 20-60 approx).
I quickly focussed on the base game for PBEM, simply to minimise the interruptions and re-dos inherent in the Tactician Deck. However, I have played a couple of full team Epic / LGB games that went (to me) surprisingly well - probably due to commitment of all the players, giving prompt turnaround and supporting the logs with email communication.
Simultaneous games is a great idea; I do this with one player. In that case, we play each side in parallel and then start a new scenario as soon as one of those games ends. Playing games in parallel would make a PBEM version of Mark McG's more intensive "OT" format more feasible.
Trust aside, you might want to consider using an external die rolling system to remove the most obvious temptation. As you are using Discord, the
Rolisteam Discord bot
would be a good choice (a macro for the CCN die is easily created). Your tournament conventions/rules could address how to coordinate between log files and die-rolling. e.g. one channel per active game, and a reference that links bot die rolls with the log file.
In my games, we try to cover alternate forks where they are obvious and allow completion of the turn (usually). This applies to squaring, r&R, & retreat. All the opponent needs to do in these circumstances is open a log file at the appropriate point, start re-playing the incoming log into their new log and comment yes or no before the event outcome is displayed. Rarely, an opponent might want to decline a battle-back, using the same method. For complex situations I often find it easier to return the log file for a decision, but for players who are attentive on the game's Discord channel, a lot of these situations could be resolved in semi-realtime. Some changes will compromise subsequent decisions in the log file. First Strike is the obvious one; e.g. results might prompt the active player to take advantage of banners for example, so they need to make that decision without knowledge of the alternative outcome. Any situation which opens up the *possibility* of a changed decision by a player who knows the downstream log has to invalidate that remaining part of the logfile - including card draw. First Strike will often fall into this category anyway, if you are to stick to the strict card draw order ruled by the card.
Hope some of this helps. Good luck!