Morrisons value foam bath, seriously. 18p for a 1L bottle, does about six refills. Works a treat.
While you've bought the stuff, refill handwash bottles with it too. Works just as well, fine even on sensitive skin.
I've tried screenwash in the past and it just smears. Detergent actually clears the screen. You only need the alcoholic stuff in the winter. Even then it usually freezes after spraying anyway, so pointless.
I don't believe that diluted detergent would harm the blades. It certainly won't kill them before they're worn out anyway. Windscreens stopped having rubber seals about 20 years ago.