First, you’ll want to use this as your starting point (the harm default variant). The way you have things now the targeting priorities are weird.
Get rid of the [mod:alt,@player] HELP_SPELL;
condition set because you can’t Intervene yourself.
Then you just slap [mod:shift] Shield Charge;
on the front of it.
#showtooltip
/cancelaura Bladestorm
/cast [mod:shift] Shield Charge; [@mouseover,harm,nodead] Charge; [@mouseover,help,nodead] [help] Intervene; Charge
If you want to be able to mouseover Shield Charge:
#showtooltip
/cancelaura Bladestorm
/cast [mod:shift,@mouseover,harm,nodead] [mod:shift] Shield Charge; [@mouseover,harm,nodead] Charge; [@mouseover,help,nodead] [help] Intervene; Charge