I've never done Hollusions outdoors but if I do, I'll try this stuff first:
https://www.mosquitocurtains.com/application-halloween-projection-screens
With all Hollusions you want as much of your viewing area as possible covered so the audience has no other frame of reference. No matter how sheer the material, there is still a chance it can be detected if you have a portion of the viewing area covered and an adjacent area not.
Hot spots and bleed-through are dealt with in a simple fashion - keep it out of the spectators' line of sight! You might rear project with the projector in a location the viewer can't see (off to the side, low behind a prop, etc.) The bleed-through would need to land on, say, the back of some prop (a wall, a crypt, a bush, etc) or fly over their heads if the projector is low to the ground.
You could also front project and hide the projector behind or inside a prop, sign, wall, etc In such a case I would try to disguise bleed-through in one of three ways. (1) Have the bleed-through land in a location or on a prop that can not be seen by the viewer. (2) Project where the area behind the screen travels out over a large distance. The farther the projection travels, the larger and fainter it gets, until at some point it becomes unidentifiable as a coherent picture. (3) Disguise the bleed-through by a "fire/ice" light or other random visual background.
Yes, you need your background illuminated, at least to some degree, to achieve the "hologram" effect. If it's dark in the background, there's no way to tell that you can see through the character because there's nothing there to see!