I too use 70% alcohol in a spray bottle or Safer's Insecticidal Soap. I tried mixing my own and although it didn't harm my plants (I used Castile liquid soap), it also didn't harm the bugs.
The good thing about both alcohol and Insecticidal Soap is, because of the way it works, the bugs never build up a resistance to it. I usually rinse my plants after treatment but more because I want to wash the dead bugs away. Also, neither of these remedies will kill most beetles, caterpillars and slugs. Weirdly enough, Insecticidal Soap will kill ants though.
For hard scale, something with Dinotefuran as the active ingredient. I use it sparingly and as a last resort (sometimes I toss badly infested plants rather than play with the big guns). Its a systemic so I would never use it outside as its a broad spectrum pesticide and will kill anything feeding on any part of the plant (bees and butterflies included).
There is nothing you can spray on the outside of a leaf to kill leafminers. If you see they are gone after you spray, it was probably just time for them to morph into their adult form and fly away. Or, you damaged the leaves so badly, you killed the leafminers too.
Hydrogen Peroxide should never be used directly on a plant. Its a caustic bleaching agent and will react with anything containing moisture. If you see a fizzle when you spray, your plants will suffer for it at the cellular level. If there's no fizzle, the Hydrogen Peroxide has been reduced to water. It won't harm your plants but won't help either.
