1. The problem with style sheets is that they are using / have to use relative paths in image URLs, and Qt doesn't resolve that automatically. Since you're strictly using relative paths (relative to the path where qmc2-mame.exe resides, I suppose) you must place the style-sheet in the same folder as qmc2-mame.exe.

2. Yes. See http://doc.qt.io/qt-4.8/stylesheet-reference.html (and search for "font").

Note that some objects have been renamed recently (object names that are directly addressed from the style sheets) and so I suggest you wait for the next release before you make your own changes. The style sheets on the QMC2 homepage have already been updated accordingly (http://qmc2.batcom-it.net/index.php/style-sheets/). QMC2 0.57 should be released next week.

3. What's set as "Configuration path" in the arcade mode setup dialog?

4. I'm not sure I understand that question... the device mapper maps files to devices upon launching the emulator. Mapping files later during emulation only works through the emulator's TAB menu.

There is no way in QMC2 (yet) to reconfigure the emulator when it's running.

5. The "memcard" option has been removed in MAME several versions ago. It's interesting though that you're not the first to ask smile.

A mind is like a parachute. It doesn't work unless it's open. [Frank Zappa]