Five, one of them is Augustus.

Mithraism was not an imperial cult, it was a military one. It thrived under several emperors. We know little about it as its rites were secret. We do know that it originated in ancient Persia and that the god Mithras was always seen killing a bull.

Yes

Such as,

The Mages Guild

The Fighters Guild

And several other things such as the Imperial Legion or Imperial Cult and many more.

The Persians were very tolerant and did not interfere in the varieties of polytheism which existed. The imperial cult of Zoroastrianism remained just that.

Augustus himself did not create any imperial cult. The worship of Julius Caesar had alredy been established before he took office and he kept to the standard Roman gods, Apollo being his favorite. However after his death, others established what could be called the cult of the emperors by deifying him.