OK, the next session is coming soon, so I am in a hurry (so laaazy, am I).
Here are my last try at this *@%%§§ Map :

Version with no transparency at opening on the central shaft :
Tomb_of_Blood_Everflowing_level_2_Version_1_1.jpg

And then with transparency revealing the openings :

Tomb_of_Blood_Everflowing_level_2_Version_1_1_Transparence.jpg
And the Objets to hide an theam reveal the areas in MapTool :
Tomb_of_Blood_Everflowing_level_2_Version_1_1_Grotte_1.jpgTomb_of_Blood_Everflowing_level_2_Version_1_1_Grotte_3.jpgTomb_of_Blood_Everflowing_level_2_Version_1_1_Grotte_2.jpg

Is it better?

I won't be able to change it prior the next session but I nonetheless will be glad to learn more...