Try to get a Mercedes cap or at least a new gasket to make yours fit tighter. Can't speak to #3 as my '80s diesels have the later generation glowplug system without the 'round thingy'.
1/4 inch play doesn't sound excessive for the AC belt. As the diesels have a lot of vibration, check for any play in the AC compressor mounting bolts and brackets. Look for loose or missing bolts, and (if applicable) damaged or disintegrated rubber mounting bushings. Also check for a loose brace from the bottom of the compressor bracket to the left-side motor-mount pad on the engine block. The brace could even be missing as sometimes lazy 'mechanics' don't reinstall it after replacing an alternator or waterpump.
If everything at the compressor mounting checks out OK, the AC belt could be glazed or burnt.

Change oil and filter. Use a diesel-rated oil, such as Shell Rotella 15w40. If you change the fuel filters, you'll need to bleed the diesel injection with the hand primer on the injection pump. Look for details in your 240D owners manual.

The Haynes repair manuals are better than nothing but I find they have too many omissions and inaccuracies for serious DIY wrenching. Try to find some factory 240D repair manuals on craigslist or eBay.

Happy Motoring, Mark

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DrDKW