Macro is
#showtooltip
/cast [nomod] Crusade
/cast [mod:shift, target=mouseover] Lay on Hands
How can I make it target@mouseover else if no target@mouseover, target=player?
/cast [mod:shift@mouseover][@player,exists,nodead] Lay on Hands
This didn’t work 
When creating @unit macros with fallback you need to include an existence check: exists, help, harm. If you don’t then the game assumes you always have a valid @unit and won’t move to the next test.
#showtooltip
/cast [mod:shift,@mouseover,help,nodead] [mod:shift,@target] Lay on Hands; Crusade
In this macro I’ve also explicitly called @target for the fallback (with no existence checks) this will make it so it won’t fallback to auto self cast.