Using photographs of these copies and those of Alfred and Saunière I compared them in Photoshop to see if I could ascertain the true identity of this relief.

Image of Alfred used for the comparison with background removed and turned to match the angle of the head on the tombstone copy photograph shown below.

Tombstone Image to use for comparison

Alfred has been made slightly transparent and laid on top of the tombstone copy image. Apart from decreasing the size of the chin slightly and the back of the head, it is an almost perfect match. (Changes can be seen in the 2 small Alfred photos shown above)

Bérenger Saunière and Alfred Saunière
As you can see the hairline of Alfred matches the tombstone whereas Saunière's does not.

Tombstone Copy Close-up
As it is almost a certainty that the image is of Alfred we have to wonder how this mistake came about. The tombstone would have been copied from a photograph, but who gave it to the stone cutter? Marie would have known the difference between the two so it could not have been her, and presumably everyone else in the village would know what Saunière looked like.

This photograph of Alfred shown above, that used to stand in the villa Bethania dining room, seems to be the photograph the tombstone bas-relief was carved from.
This would not be the first time Alfred would be mistaken for his brother, newspapers and even Gérard de Sède in his books would label Alfred as his brother Bérenger. As they both had a similar appearance perhaps we can see why those not personally knowing the brothers would have made this mistake. If we too did not have at our disposal today the luxury of all the photographs to compare them to each other and the tombstone we would probably make the same mistake.
Visit these links to read more about Sauniere and Alfred
Secret of the Abbe's
Alfred Sauniere
Saunière, Freemason?
Rennes-le-Chateau Mystery Timeline - Part 1