NO! Don't put peroxide on your plants to kill powder mildew! Look at that article the words "probably best"!! Do not do that especially ever in any concentrated dosages.
And NO do not lower humidity lower than 40%. That will drastically negatively affect the transpiration and metabolizing rate of your plants.
Humidity between 45-55 is optimal.
And NO! don't increase air circulation when you have visible powder mildew problems. That would make even more of the powder mildew airborne and spread the problem like crazy.
If you want a 100% cure for powder mildew buy a product called NOVA. It will get ride of it completely usually with only one use if you completely spray the infected areas. On the odd occasion it will take two doses.
Your temps are a bit low and that could have an effect on powder mildew if those temps are constant and if your night time (lights off) temps are even lower.