Hooking up a laptop to 12V alone might work. The Lilliput and 3.5"HDD both draw about 1.5A from the M1-ATX. That's only 0.5A for the laptop -> NOT enough.
Any low wattage inverter would work because I see that you only use it to power the soundcard. Get anything that has wattage more than the power consumption of the soundcard then you should be ok.
BTW, your design is too complicated. Why so many stuffs? (especially using both inverter and M1-ATX). For the price of both, you could buy an Opus 150W. I think the less the better.