- PLA as a material is at least food safe (though food safe and body safe aren't the same). But! The material passes through nozzles which probably have lead in them etc. even if that's of negligible consequence though the main issue with making something body safe is the layer lines, which retain moisture and breed bacteria.

- yes, concave surfaces are dang near impossible to smooth.

- plenty of household release agents don't work in a silicone to silicone mould, or impede the cure of platinum cure silicone (the body safe kind), and if you bugger it up you're just wasted the mould and the dildo's worth of silicone which is $$$.

- I just used PLA. the beeswax doesn't touch it for long, you literally pour it in and immediately back out. Because the print is hollow on the inside and because beeswax has such a low specific heat capacity it cools instantly, and this all works, my pics are proof.

- silicone doesn't bond to PLA, no, but you need to smooth the layer lines, which brings us back to point 1....