Yes i understand.
i didn't manage to reproduce this pb, i tried many things.
Fresh install of contao and merconis + default merconis theme, is working well.
the only thing i didn't do, is fresh install contao+merconis and redoing theme "piece by piece" because not enough time (my theme is quite big, lots of files, modules, layout).
i don't understand why the params added by merconis search and the usage of aliases in url are messing up,
if i uncheck "Use different layouts in product details" in the site structure, i have a full 404 page, if i'm checking it, i'v got a mixed page (breacrumbs show "not found" but product details are here).
i posted again here, because your knowledge in contao is maybe a way better than mine, so i hoped you'll have a new idea :p