During my adventures in Egypt, I've only found a total of two spawning places for Mummitomium.
The first is by the mine you mentioned, but note that sometimes the Mummitomium actually spawns inside the real mine-decoration that is there
. So occasionally, you'll have to tinker with the camera angle quite a lot before you can see and click on it.
The other spot is on the grassy part by the river, near where the river goes out to a big sea.
But both spawns take a few days, so trying the relic merchant is probably a good idea too.
The adventure in Egypt that needs Mummitomium actually requires TWO metals, and not three. I've never had a problem with the adventure since the map tags for Mummitomium has always showed me to these two places, and I've always been able to collect from them.
EDIT: and Hi people, new to the forum here!