You dont need an anti virus app on a phone. The only apps that are sketchy, are ones from china/japan, but if you are smart, you will just avoid those apps all together in the Market. Only install apps that have more than a few thousand people using them, dont install ones that have less than 100.

I havent used that app, but I'm sure you can disable the anti-virus features, and still use that scream feature.

Call sprint customer service, tell them the problem you are having. They will probably "refresh" your network, or maybe even just re-activate the phone. If that doesnt work, you should try a hard reset.

 

Can't help you on the first item, I don't use AV on my phone nor do I think it's necessary.
I tried to cause a signal drop as you described, but was thankfully unsuccessful. I'm in a decent signal strength area. I've had very little signal problems with this phone in general, *maybe* 2 dropped calls out of several hundred in 4 months. Maybe an antennae problem with your phone? It's not unheard of.