You use the unused phones to mining. Why do you care about virus? You can take it to phone service shop to check it. Or simply download an antivirus app to scan it.
Yes, the same point. If you use them just for mining, then no worries.
Have never tried mining with a phone. Can you do it with your own phone you use daily? Will it slow the phone down too much? Sorry, not really familiar with mobile phone mining.