If you have yourself targeted, then yes [help] would cast Renew and you would never cast PW: Shield. Only if you have NOBODY targeted would the PW: Shield go off since neither [help] nor [harm] are valid. I just tested it on my pally while sitting kinda AFK in wintergrasp, my macro was:

#showtooltip
/cast [harm] Seal of Light
/cast [help] Flash of Light
/cast [target=player] Holy Light


if I target an enemy, Seal of Light casts, no problem. If I target a friendly player, Flash of Light casts. If I target nobody, Holy Light casts on myself. If I target myself, Flash of Light casts on myself, since [help] when target is myself evaluates as true.

So imo just always target nobody when you want it to be you and you will be able to do what you want. Otherwise you will have to do things with /castrandom and commas and hope it drops through randomly to abuse the GCD logic.