Yeah the phone should be dust resistant, so that there is dust inside is concerning and would make me think the phone has issues with ingress. Getting the repair completed should actually resolve that, since they replace the adhesive that gives it the IP rating. If this is an iPhone 14 or 14 Plus you could also ask them to reseal both the front and back glass during the repair. It’ll probably take a bit longer but they’d likely understand given dust has gotten in.

Then you go get your phone repaired.

The agent is correct that if there is an issue with your device it should be fixed before AppleCare+ is added.

However, debris under the lens is a warrantable issue provided there’s no damage to the device. You can use your warranty to have your phone repaired. If this is an iPhone 12 or newer, it will not be replaced, it will be repaired. That is also true if you had AppleCare+.

Take it to an Apple Store, get it repaired, then add AppleCare+.

Touch screen doesn’t work properly and is registering false presses, that’s all that’s happening. If the flashlight is coming on unexpectedly too then it’s likely the phone is liquid damaged because having both of those things fail at the same time is unlikely.

Edit to add: the forums are right, this is not someone remotely using the phone.