Regarding the planets, Jupiter might be visible at the moment but it’s at a low angle. It follows the moon-rise, rising in the E-SE around 9pm and appearing to the south by about midnight.
Not quite. Jupiter will rise at around midnight in the NE and move towards the S then SW through the night. It’s a very obvious object in the sky and is quite high in the sky in the early hours. It looks stunning through a telescope. It’s followed quite closely by Mars which is pretty difficult to identify if you don’t know what to look for but again is obvious in a scope. Venus is very bright but very close to the sun at sunset and sets shortly after. A few weeks ago it looked very bright in the SW sunset. Uranus is also visible through a scope – I found it a few weeks ago and I believe Neptune is out there but I can’t find the bleeding thing!
So yes there are plenty of planets out there and visible.
In answer to the original OP – the obvious reply is that any of the numerous amateur astronomers like myself may have noticed a new planet and maybe talking about it on the relevant forums. It would take a monumental amount of censorship to cover it up. And seeing as we can spot the Galilean moons of Jupiter, I’m sure that someone would have noticed Planet X or whatever Loony is on about.