Hello Lissbirds and welcome to the forum!
I wouldn't spray it on the flowers. For flowers and buds I use wormtea and it does a pretty good job and doesn't harm blooms or buds. As for the plant itself submerging the pot and letting it soak is best. This will not hurt the plant. Let it soak for a few. I've even let mine soak for hours without any ill effects. This also goes for my plants that are in S/H.
You do have to rinse the medium afterwards. Spray the plant for what buggers is on there, except for the buds and blooms, and rinse the medium when finished. You don't have to wash the plant off.
Hope this helps.