I'm not convinced that its that easy. God's purpose was only to destroy the world of mankind. At the time that it occurred that may well have been a limited area. Man had not spread abroad as he did after the flood (Gen 10).
The descriptions are written from Noah's point of view. 'Under the whole heavens' indicated the heavens as he saw them. The mountain tops that were covered were those within his range, and in those days they may well have been lower than today.
Thus a massive, but not worldwide flood would fit the Biblical facts. Why cover Australia and America if man had not reached there? It was caused by the 'fountains of the great deep opening up'. Who knows what convulsions of the earth caused this? The rain only had a secondary part to play, although more apparent to Noah. It is quite possible that it was linked with the ice age in some way.
The pre-Flood world was very different to the post-Flood world. For example, we know continental drift happened and that the earth was one super-continent, but if you go by the Bible the continental separation can't have happened 100 or so million years ago, but rather, the only time it could've happened, without killing every land-dwelling person and animal, is during the Great Flood of Noah's time. That also answers your question about Australia and America. As for the Ice Age. There was only one and it was a result of the Great Flood. Biblical creationists believe it lasted for up to 700 years, following the Flood.
Please tell me how the mountain ranges were only covered from Noah's perspective? Water finds its own level.
That's a heck of a meniscus effect! It's just not possible. That said, you're right that the mountain ranges would've been far lower than today, because the higher mountains were formed during the Flood and a little after. And yes, you're right about the fountains of the great deep having the biggest role to play in the Flood.
Genesis 6-9, read in context, with all of the elements that make up the Flood account, plus with what Jesus says and some of His disciples say concerning the Flood, there's absolutely no way to interpret the Great Flood as a local Flood or just an allegory piece. No way.