For Miami I would recommend 20W-50 mineral oil year round, but it probably won't put a big dent in your oil consumption. If you're only driving 100 miles per week, you could just change it once per year. Consider new valve seals. A decent independent shop should do the job for about $300.
It sounds like short trips and relatively high oil consumption may be resulting in some deposit buildup, but unless the engine misfires, it's not a big deal. The Bosch H9DC is THE correct plug and they should be pregapped at .032", which is Mercedes' recommendation, but plug gap should always be checked prior to installation.
Before I refreshed the cylinder head on my Cosworth Vega last year it was consuming a quart of oil every 100 miles, but the plugs (HR9DC - same as the 103 engine except for resistors) did not foul out though they did build up a lot of oil ash deposits on the insulators.
The new guides and seals reduced oil consumption to about a quart every 5000.