Question about Digitus RJ45 T-adapter

No, that won’t work. You always need 2 of these items. They are used in scenarios where for example a single cat-5 Ethernet cable exists through RJ45 wall sockets connecting two locations. At each end you connect one of these Y-connectors which allows for 2 LAN cables to converge and get transmitted through this single Ethernet cable. At the other end this combined signal is split via the second Y-adapter back into separate cables. But be aware, both cables have to be wired in the same way. Many but not all RJ45 plugs are wired in the same way. If the wiring differs between the two, then this setup won’t work either. Also take note that splitting LAN cables like this reduces the speed to a maximum of 10Mbit/s.

No, that is not possible.
This adapter (you always need 2) is designed to operate e.g. 2 PCs on an 8-wire LAN cable. It splits the 8 wires into 2x Ethernet 100Mbit/s.

Probably yes, but the LAN transmission does not work for me at the same time. In my opinion, it's a solution for telephone and Internet, because the box in the wall is crucial.