tiShare

You can select multiple items with SHIFT + CLICK in tiShare Desktop. In tiShare Mobile, You can do the same thing by long-pressing checkboxs.

Yes, but you should use v0.6.0 or higher. That is the first version that shares files using TLS, a widely adopted cryptographic protocol that provides data security over the networks.
However, to mitigate impersonation attacks, check if the fingerprint of the other on your screen actually matches that of the other device. We do not recommend and do not take responsibility using tiShare versions below v0.6.0 in public and untrusted networks, such as coffee shop Wi-Fi routers. For security, please update your tiShare apps.

In order for two devices to communicate, one device needs to open itself. Press the button located at main screen or Device tab to open the device to the network, and press Scan button on the other device. The opened device will show up on the list.

Sometimes unknown errors happen so devices are not detected. Close and open again. Or open another device to the network. Also, firewall may be blocking accesses to the device. Check if tiShare is on exception list.