I can’t find rules support, but I recall that injury and helmet off by the defense requires the next play clock to be set to 40 in all cases. Maybe, someone else can dig up support.
Not always. 3-3-9-b (helmet off) says this happens if the helmet off is the only reason the clock was stopped (other than an injury to a player of the same team). If there was one or more other reasons the game clock was stopped e.g. a flag on the play, then the play clock is set to whatever that other reason or reasons cause it to be set to.
Interestingly, the way that 3-3-9-b is worded has an implication that the same should be the case for injuries (as the wording suggests that the two situations - helmet off and injury - should be treated similarly), but the wording relating to defensive injuries doesn't make reference to other reasons the clock was stopped.
Flag on the play plus B helmet comes off - 25s play clock.
Flag on the play plus B injury - the rules as written are unclear. 3-2-4-c-1 says 25, 3-3-5-e says 40.