I got these from a greenhouse which I later learned was having issues with powdery mildew. I don't see any signs of mildew growth on the three plants I bought (I'm not even sure that haworthias can get it).

What's the best way to ensure that I've killed all the spores so these plants will be ready to leave quarantine? All advice I've found online concerns powdery mildew GROWTH on plants, so I'm not sure it's applicable to hidden spores.

I have hydrogen peroxide and copper fungicide on hand but I can probably get some other chemical if necessary. My current plan is to:

1. Spray the outside of the pot, the entire plant, and the top of the soil with hydrogen peroxide

2. Wait 1 day

3. Unpot the plants. Spray the plant again with peroxide. Spray every bit of exposed soil with peroxide

4. Shake off as much old soil as possible. Spray roots with peroxide (maybe more diluted). Wait a bit. Rinse roots with water. Repot in fresh pots in gritty soil.

5. Keep them in quarantine for up to a month while I wait for signs of mildew. Spray around once per week with peroxide if the plants didn't react badly to it the first few times.

6. Spray with copper fungicide and put them with my other plants.

  
I'm not sure if this is overkill/underkill so I'd appreciate some guidance.

These plants likely have powdery mildew spores on them and in their soil. What's the best way to kill the spores so I don't infect the rest of my plants when these leave quarantine?