Answer 1: I was born and raised in Sydney and did a lot of herping from when I was a child until I moved to WA at the age of 23. All the Litoria ewingii I came across were on their own. In contrast, a species like L. fallax .I have only ever seen as a group. Based on that I am confident a single individual will be fine.
Answer 2: Pinhead crickets are likely to be too small. Frogs will eat pretty much anything living they can fit in their mouths. Crickets the width of the head of the frog would be a more appropriate size. In nature these frogs eat mainly flying insect such as flies, moths and mosquitoes, which they are very adept at catching. It is therefore very important to provide variety in the diet of frogs. It is also important to avoid things that contain particular high levels of protein or fat, such as pinkies. Frogs are not designed to cope with a diet high in either protein or fat, and this can result in fatal damage to their internal organs. Those insects or larvae with high fat content, such as mealworms, giant mealworms, silk worms and black soldier fly larvae, need to be provided sparingly. A iight based home insect trap used at night during the warmer weather is an excellent way to collect food items. An the same is true for a fly trap does not drown the flies it traps. Woodies (Speckled feeder roaches) of appropriate size are also a good food item. Other suitable food items, such as grasshoppers, can be collected by running a butterfly net through sizeably long grass during yhe warmer months.
Answer 3: There is a range of substrates that are suitable. Unsuitable substrate are things like sharp rocks or gravel, rocks of a size that may be accidently ingested but not readily passed through, like sand would be, substrates that stick to frog’s skin, and substrates that can go mouldy when kept continually kept damp. You can use soil mixed with coir fibre and/or sphagnum moss, I desired can be covered with large aged pine bark (used to pot many orchid species) or aged leaf litter that has lost most of volatile oils. Alternatively smooth river stones and sizeable pebbles are acceptable and shallow depth of water can be maintained below them to provide good humidity.
Litoria ewingii was introduced into New Zealand in 1875 and is now naturalised through both islands. Following is a New Zealand info sheet on it. I have also included a on sheet the common green tree frog, which has essentially the same requirements, except L. ewingii does not need any lighting
Do provide plenty of vertical, slanted and near horizontal elevated perches for this species to utilise. If using live plants, species suitable as indoor house plants, that have slid stem and broad leaves, such as Philodendron erubescens and Monstera deliciosa small leaved form. A suitable vine to grow over the perches is Epipremnum aureum, otherwise known as Devil’s Ivy or Pothos.
https://www.reptiles.org.nz/herpetofauna/non-native/litoria-ewingii
https://www.reptiles.org.nz/herpetofauna/non-native/litoria-ewingii
