And
4. every island (mountains under the sea) fled away, and
The use of the word "fled" reminds us how quickly this occurs. See Revelation 9:6 and Revelation 12:6 for the other times when things or people fled.
This verse implies a complete reversal of the earth's tectonic plate movements. Over the years since creation, land masses slid over each other (sometimes violently with earthquakes) to form mountain ranges, valleys, and islands. So as part of the reclamation of the title deed for the earth as embodied by the scroll in chapter 5, it is entirely appropriate to "unslide" the earth's tectonic plates to restore the earth to its pristine creation state. All this is in preparation for the millennial reign.
5. the mountains were not found.
Mountains have been the place where God meets mankind (Exodus 19:1-9; Exodus 19:16-25; Isaiah 2:1-4; and Matthew 17:1-13). However, during the millennium, God/Christ will again dwell among men and will not speak to them from a mountain top. That is not to say, there won't be "mountains" around during the millennium. I suspect that there will be a mound at Jerusalem as that city will be the capital of the whole earth during that period.
Evil men have used mountains to hide in (Revelation 6:15-16-17). However, at this time (end of the tribulation period), there is no need for men to hide. Those who received the mark of the beast are clearly identified, and into the millennium, society will expect God to walk among them just like God walked in the Garden of Eden with Adam and Eve (pre Genesis 3). Curiously, even as in Genesis 3, Satan will be released after the 1,000 years to try to tempt mankind again (Revelation 20:7-10).
An aside: If it is a solar recurrent micronova as described in the Revelation 16:8 update, then geologic records indicate a tremendous tsunami probably will inundate the world (cause: the movement of icecaps to find their centrifugal equilibrium points or some such). Likewise, mountains will be moved around as floating islands on magma flows. The reader can surmise how these position changes will be verified (if the mountain hasn't been swallowed up in the flow).