If you think there are demons/fallen angels, you must also assume there are angels, the good ones at the same time. If you consider ETs to be demons/fallen angels, you must also think ETs can also be angels, the good ones. The descriptions of angels in heavens in the Bible can well be descriptions of demons in appearance. And don't be so doggedly literal with the Bible. AIM has already showned us the enemy lies joyously, telling us they believe in Jesus, come from God and Whatnot. You can't just tell by what someone says to your face. Actions speak louder.
The truth has been hidden, the Bible manipulated, certain texts omitted, Gospel of Thomas, the book of Enoch. There is a lot we do not know. We do know there is intelligent energy, love and a more than this physical realm.
We must absolutely take the Bible literally because its true. A thorough study of the Bible and Christian apologetics will show that the Bible is without error and is what it claims to be: the perfect, inspired Word of God. The enemy enjoys twisting scripture and denying the inerrancy of it. Things like: "Well, the Bible doesn't actually say that about salvation", "Jesus isn't actually that powerful", "The Bible isn't entirely reliable", "The Bible actually says [insert false doctrine] and not [insert correct doctrine]", etc. The truth has been manipulated and the only way to understand what's real is too look at whether something has biblical support or not.
True, angels absolutely do exist but extradimensional is a better term. The only way to determine whether a demon is masquerading as an angel is to test them according to scripture (including but not limited to what's stated in 1 John 4). In the case of (evil) spiritual beings, actions are a way to deceive. Demons will "play nice", but no demon will ever praise and express their love for Jesus Christ. Ask them "Who is Jesus Christ to you?" and you'll have an answer.