What do you think would Ethereum sustain this pressure and retain its leading spot?
to maintain the position seems to be quite difficult now, but ethereum can certainly do it
How would the Proof of Stake, if implemented in time, work for Ethereum?
POS is indeed very attractive to investors. but I see ethereum still haven't decided whether to use this consensus or not. if indeed ethereum is serious about using POS, maybe there will be a big change in the market. many people will hold ethereum in large quantities. and this can trigger a big pump. but if someday there is an ethereum dump that will experience the most severe fall, because there will be many people who will sell ethereum.
Would Ethereum solve their scalability issue?
for now I'm still not sure.
Does Ethereum decentralization give it an edge over other centralized smart contract platforms?
not really, there are actually many similar platforms that are better than ethereum. but ethereum has a royal community so it looks better.