In all cases, follow country regulations, AFM and/or company procedures. In their absence, consider the following.
1. Depends on the fms. Some allow you to quickly load an approach at the departure airfield, so use of a secondary flight plan may not be necessary
2. Yes. An overweight landing is generally considered appropriate in a situation such as an engine failure and return to departure. Of course, maintenance will need to conduct their checks afterwards.
3. If the chart is for a specific runway and/or flap configuration, select another runway or flap configuration. Use max allowed thrust. Use any headwind available to improve take off and climb performance (gradient). Consider effect of runway slope. Off load freight to decrease weight. Wait until the temperature cools enough or the altimeter setting rises enough. Basically, you can't go unless your numbers meet the requirements
4. FMA CATIII single, or CATIII dual: No idea