Will using Redgard over a cement shower pan cause a mold sandwich if there are weep holes underneath?
I don't think I'd bother.

pvc glue melts the plastic together.  It is used in plumbing with no leak issues.

If you do red guard it, I would not be concerned about a mold sandwich.  Even if mold did somehow get enough food and water and did grow in the deck mud, how would mold spores get out of this sandwich?  

How would mold growing inside deck mud - under tiles - that receives the same food and water be less worse than a mold sandwich?  Is the opinion that it can dry easier make it better?