Let's say trooper T1 and T2 are moving together in a order (Fireteam, peripheral, coordinated or so). Trooper T1 enters to the mine's trigger area with the first short order. Trooper T2 stays behind the wall for the first short order, so T2 is not a valid target for a mine yet. Mine declares to blow off, and at the second short order, trooper T2 moves into the mine's trigger area. Does mine have to place the template only against T1? Or the mine can place template against T2 who is not in a trigger area when it declares to detonate? Assume there's no angle that a template can hit both troopers. I think if mine has to blow off only against T1, it is against the all at once principle as it distinguishes the first short order and the second short order. However, if the mine's template can be placed on the T2, then it conflicts with "mine must detonate and template must be placed when the attack is declared".
Mines do not declare skills, they trigger mechanically, meaning they have no rule mechanic to delay or hedge their bets. So you check the trigger area and then the owning player picks one unit that has entered the trigger area, and any unit that hasn't entered the trigger area is ineligible.
