I think the reality is not quite in line with what you think
Higher prices usually mean there is less supply, that's the reason behind prices rising (apart from demand inflating, of course). The higher prices we see today are in great degree due to July reward halving now taking full effect (i.e. supply side contracting). In other words, with prices rising markets become thinner in general, and it is even truer in case of Bitcoin (since folks right now prefer holding to active trading and selling). So your millions of dollars will cause higher volatility (in both relative and absolute terms) with high prices and low supply in contrast with low prices and high supply
I think youre wrong. Lets pretend BTC reached 10,000 US dollars. Are you saying that those super high BTC markets are now so thin, a slight push up will bring the price easily to 15,000 dollars with a less than a million in volume? Explain to me how that is so
In fact, I have explained it many times already, on numerous occasions
And the explanation is pretty straightforward. You obviously don't take into account two things. First, the supply of new Bitcoins is limited, and it is not just limited but it is actually diminishing due to halvings, coins lost for good, and folks stashing their coins away. Second, if the price rises, it basically means two things again, i.e. that the supply of bitcoins diminishes (which is consistent with the first point) and that the demand for bitcoins increases. Any way you look at it, you have to agree that the Bitcoin market is necessarily thinning in any of these cases. Given that, it becomes more susceptible to heavier price fluctuations simply because the same number of coins can now move the price stronger (or less number can move it for the same change in the price, in relative terms)