No you dont need moss, if ingested with very high probability might cause impaction.

Lecroixe, you must be a lucky one that have working analog gauge they are very unreliable and not once were a cause of frogs death. I woundt trust them or recommend anyone to, unless you have a backup to constantly check up their readings.

Plastic plants wont cause any injuries they are actually one of the best to use to pacman frogs, you can sanitize them if needs to, silk are good too, but if any problems occurs, it is hard to disinfect them the way you might need to, in that case you will need to grow them away. The only risk with plastic plants might be if it is old and disintegrating, pacman might ingest a piece of it if fed close to it, but in all years i've dealing with animals i've heard only one case like that.
Live are the worst for pacman frogs, i wont go over why, i recently wrote about it somewhere here.

Powder - in reptile section of the store look for any powder ( in small bottles) that would say multivitamins and another bottle that says ca with vit d3.

Water - yes you can use bottled spring water or use water conditioner for your tap water. I very rarely use spring water, it is too expensive to use in quantities i need. Small bottle of prime lasts about 6 month in my household.

Care sheet - Frog Forum - Pacman and Horned Frogs - Ceratophrys - Care and Breeding