1. 4 mm² for each of the 3 wires
2. C25 or C20
3. Direct wiring with disconnect switch, commando plug or round pin 15A socket + plug

If TS's thread picks up steam, you are going to have a lot of differing opinions on what you are asking.
But my personal opinion is that if you understand the safety issues, limitations, and the repercussions, then your extension is possible. It is entirely your responsibility at the end of the day anyway.

You could make sure only one appliance is used at a time by using 2 plugs and 1 socket, so that only one appliance can be used concurrently. Use quality 15A plugs and sockets only, so that an appliance cannot be inadvertently plugged into another socket. Or use a permanently attached change over switch (but it's a bit ugly)