portals, opening portals, is an occult practice. period. and you are right, God does not need us to open portals and there is nothing in scripture indicating we should practice or attempt to practice this
it sounds to me, regarding the scripture in Revelation you quote, that the demons currently bound cannot have access but they are granted access at the time of God's command. I'm not seeing that as a portal though, but it is definitely God that enacts the release of the creatures
see, again, going back to Revelation, they are GIVEN power. God gives them this power.
this portal business is not of God. can demons take on the form of being a godly spirit? well, the devil comes as an angel of light and yes dark beings can appear to be of God
I don't want to say more, but this portal thing is not of God