Hello calvin here,
No need for deeper discussions that take us further away from God's word.
I do indeed recognise the possibility that Everest did not exist pre flood.
One estimate of current atmospheric water is 3,094 cubic miles (USGS)
That is a lot of water. At the time of the flood that volume might have been quadrupled for example, and if the Earth was at that time less hilly, along with whatever water was released by subterranean vaults (fountains of the deep),there would have been enough water to dunk the whole planet.
Gen 7:20 talks about the waters covering the mountain to depth of 15 cubits and Gen 7:24 says that the waters prevailed for 150 days. This is in agreement with Gen ch 8.
I have yet to hear a convincing theory that could account for that volume of water being contained to a local area. A mighty tall Dyke built as a cylinder might contain the waters to a localized area, but that idea and any like it are simply too ridiculous for words.
Let us just believe God's word and let Him worry about the details.