Was there any sign of leakage like dampness in the walls or curb? Being winter, the air is fairly dry, so 1/8" over 20-hours wouldn't bother me. The bubble when a toilet was flushed does bother me a bit...are you sure the shower is vented properly? The whole idea of a vent is to minimize pressure differentials in a drain's trap (vacuum is worse than pressure).
Birdbaths in the preslope are not the best thing. If you use a straightedge, how big and deep is it? Keep in mind that in a conventional shower pan, there WILL be some moisture that gets down to the liner. It won't be much, but it will get there. The reason they use deck mud is that it is porous, and that moisture will drain to the weep holes and down the drain. If it can accumulate, after a number of years, it can be the source of some swamp like smells. Takes awhile, and much more common when there's no preslope. Can you post a picture?
When you say poured your preslope, that's not easy with deck mud...What did you use? Deck mud is easy to shape and it stays there...something liquid enough to pour, probably isn't.