Good morning Guillaume,
Here is my two cents:
First, substantial action is defined in rule #35 and is to be considered only during the case of a misdeal: rule #34D, and in the case of a player that was skipped by action out-of-turn: rule #40B. Therefore, the substantial action component added to scenarios 1b, 2b, and 3b have no affect on my rulings. I would rule those scenarios the same as in the scenarios for 1a, 2a, and 3a.
In scenario 1a, player C makes an undercall in turn, which is covered in rule #39B. As this player was not facing a bet while heads up, nor facing an opening bet on a multiway round, the undercall is NOT AUTOMATICALLY ruled a full call. Instead, the ruling is left to the TD's discretion. The TD can rule that must be a full call or they may give the player the option to forfeit the undercall amount and fold. Technically, players D and E are in the same situation as player C. The TD's discretion is still in control here. It is possible that a TD would rule that player C must make a full call, while player D and E have the option of forfeiting and folding.
In scenario 2b, player C makes a raise of less than 50% of a full raise. According to rule #43A, it is ruled a call. Players D and E would also become callers under the same rule.
In scenario 3b, player C makes a raise of more than 50% of a full raise. According to rule #43A, it must be made a full raise of 20,000. Therefore, I would rule that players D and E have made undercalls while not facing an opening bet during this multiway round (rule #39B). I would give them the option of forfeiting 16,000 and folding or calling the 20,000 bet of player C.
Regards, B~