Hello & congrats on your new frog!

No way to tell it's age unless you know it's egg laid date. Size is not an indicator of age because you don't know what or how often it was fed and this is a crucial growing period for the frog. Offer it food nightly, it may not always eat, but offer anyway. We use nightcrawlers as a staple at this size, it provides the best nutrients in our opinion.
Nuptual pads will appear at around the age of 8 months or the size of 3 1/2" whichever comes first. Substrate is a matter of choice in the end. We use damp paper towels and/or coco fiber. The moss tends to be a health hazard if ingested. As for the little white bug, chances are it is a mite.......they happen quite often in the fiber and are usually harmless. If it is a springtail, it is fine....they clean waste and help avoid mold & fungus.

The reason not all the care sheets agree is because there is more than one method of keeping the frogs. You end up using common sense & a little bit from the sheets and do what fits best. As long as you keep the animal's best interest in mind, you will be good.

Feel free to ask any questions you have, plenty of folks here to help!