As far as canon limitations go, Danny's ghost form is very nearly as much his true form as his human side is. Under normal use of his powers, he can hold it as long as he wants to. There are a few exceptions. Complete exhaustion from overuse of his powers can force him back into his human form. Electric shocks can short circuit his powers and keep him locked in human form for several hours.
As far as intangibility and invisibility go, he is mostly shown using them for just a few seconds or minutes. He can use them for longer, but this is very tiring and he prefers not to. I'd be okay with setting a ten minute or so cap on intangibility and invisibility.
The thermos can be used to catch any ghost in Danny's world. Think of it as something similar to the ghost containment devices in Ghostbusters. I primarily listed it in inventory because Danny always carries it with him. In Arda it will likely only be able to catch Danny, seeing as the ghosts in his world are very different from ghosts in Arda.
Re: Questions / Power adjustment
As far as canon limitations go, Danny's ghost form is very nearly as much his true form as his human side is. Under normal use of his powers, he can hold it as long as he wants to. There are a few exceptions. Complete exhaustion from overuse of his powers can force him back into his human form. Electric shocks can short circuit his powers and keep him locked in human form for several hours.
As far as intangibility and invisibility go, he is mostly shown using them for just a few seconds or minutes. He can use them for longer, but this is very tiring and he prefers not to. I'd be okay with setting a ten minute or so cap on intangibility and invisibility.
The thermos can be used to catch any ghost in Danny's world. Think of it as something similar to the ghost containment devices in Ghostbusters. I primarily listed it in inventory because Danny always carries it with him. In Arda it will likely only be able to catch Danny, seeing as the ghosts in his world are very different from ghosts in Arda.