PetrW, coming back to your questions.
1. the Beast ascends into the sea (13:1) whole, including all 7 heads, and the Beast (not the head) is also cast into the lake of fire (19,20).
2. Rev. 1:1 ("what must come to pass") and 1:3 "the time is at hand" delineate apocalyptic events into the future. How is it possible that John is writing about the future but actually sees the past? Real but extinct e.g. Assyria, how does that affect the future?
What does Egypt, Assyria and Medo-Persia got to do with the price of eggs? To answer these type of questions, I always refer back to the book of Daniel. Here God presents us with a world view. First the dream image with its dfferent levels of metal. However you want to interpret it, it comes to an end with Armageddon. Then God's kingdom under Christ Jesus takes over (cf. Dan. 2:44, 45).
Within this context, one finds the vision of the four beasts. Babylon, Medo-Persia, Greece and Rome. From Rome comes the small horn, taking out three of the horns. IMO this is the British Empire, taking out its three main competitors, France, Spain and the Netherlands. The blaspheming horn becomes huge, becoming the Anglo-American Empire. Above prophecies follow the course of history, so the information from God's book of truth is reliable (cf. Dan. 10:21).
This is the background. Now we come to the book of Revelation. The Revelation beasts would follow the same pattern and I view these as a continuation of the Daniel beasts. Put them all together and you have the full picture, history in advance. The full compliment of Satan's political system is to come to an end, to make room for God's kingdom. I like to work with Biblical definitions, e.g., the antichrist. I prefer John's definition, and not those who want to make up their own definition and meaning. As to the origin of the beasts, the following helped me:
From the sea: Isaiah (27:1) describes how God will execute the elusive serpent or sea monster. According to BDBLex the mighty, multi-headed “Leviathan” symbolizes world-powers, e.g., Egypt (Ps. 74:13, 14; cf. Ezek. 32:2-12). Job (3:8, 9; 26:12, 13) refers to it as the fleeing serpent or eclipse dragon. This concept was further developed by later writers. Daniel’s four beasts come up from the sea (Dan. 7:2, 3). One of Revelation’s beasts also comes up from the sea (Rev. 13:1). What does the sea picture in these instances? Isaiah (57:20) explains: “But the wicked are like the sea that is being tossed, when it is unable to calm down, the waters of which keep tossing up seaweed and mire.” [This is the sea that will be no more (cf. Rev. 21:1).] In confirmation of this, the harlot, riding the scarlet-colored wild beast, is also described as sitting upon “many waters” (Rev. 17:1-3). Again, what is the meaning of the “many waters”? One of the angels with the seven bowls explains: “The waters that you saw, where the harlot is sitting, mean peoples and crowds and nations and tongues” (Rev. 17:1, 15).
From the earth: The false prophet is also called beast from the earth (Rev. 13:11; 16:13). What does earth mean in this instance? Nebuchadnezzar’s dream image had legs of iron, yet feet and toes mixed with iron and an earthy substance, clay. Hard and unyielding iron points to dictatorial, totalitarian regimes, whereas brittle clay pictures aging democratic structures (cf. Dan. 2:41-43). The governments represented by the false prophet, i.e., the Anglo-American empire, originating from the earth, would mainly consist of democratic elements (cf. Dan. 8:23, 24). During the First and Second World War, it would be this earth that would come to the aid of God’s woman and her children [= those of His anointed on earth] to save them from the serpent’s deluge of persecution (Rev. 12:15, 16).
From
the abyss: In the abyss = inactive. Out of the abyss =
active. The abyss can be favorably compared to the realm of the dead. Paul
contrasts “ascent into heaven” with “descent into the abyss,” but because
Christ was there, the abyss should not be conceived as an evil or demonic
realm (Rom. 10:7; cf. Luke 8:31). God allows the beast to come up out of the
abyss (of inactivity) (Rev. 17:8, 9, 11; cf. 11:7, 8). But who is it that
controls the abyss? A star (or angel) of God has the key to unlock the abyss,
unleashing a plague of locust-scorpions, the soldiers of the beast, to torment
mankind (Rev. 1:20; 9:1-3). Later Satan the Devil would be sealed in this abyss
(of inactivity) for a thousand years (Rev. 20:1-3).