It sounds like the Merlo representative was wrong. Even with Merlo coffee using their extraction method (which you've aptly described), the extraction length should be somewhere between 20-30 seconds with the dosage that you're using. 12-15 seconds is incorrect, and also inconsistent with the 'Merlo way'.
I know this from previously working at one of their roasting houses for a number of years, however there's also a video they've published demonstrating it: Fresh espresso beans = a perfect shot - YouTube
I completely agree with Muppet_Man as well; taste is of paramount importance when dealing with coffee, and similarly when experimenting with different methods

Best of luck.

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