Ok, i'll try to give some answers, altough i'm not technical. Some info about ethereum:
https://www.reddit.com/r/ethereum/comments/61y5ix/welcome_to_rethereum_the_reddit_front_page_of_the/Some of your answers are found in that link.
4. Proof of work ( ethereum will move to pos in ~1year ). POW atm keeps bitcoin from evolving. Miners, who control bitcoin's security can't agree with the economic majority ( investors ) about the way to move forward.
POW can be 51% atacked much cheaper than POS.
POW can be destroyed by quantum computers.
POW with asics caused mining, economical and political centralization of bitcoin in china. That's extremely dangerous especially since it's a communist/ almost totalitarian state.
Bitcoin has centralized governance, one protocol, one team. Ethereum has 7 different implementations of the protocol each with independent teams.
Attacking bitcoin's pow costs ~20mil ( according to vitalik ). Attacking ethereum in POS would cost hundreds of millions$ (owning the majority of staked eth in POS)
Less vulnerable to selfish mining, because if 2 blocks are equal, one is randomly selected therefore the succes chance is 50%.
5. Ethereum can do what every other cryptocurrency does, and more, because it is a platform for anything you can think of. Any application you can think of can be used and deployed on ethereum. Example, bosch will use it to fight odometer fraud
https://twitter.com/BoschGlobal/status/842378912017616897.
United Nations is using ethereum for their world food programme
https://insight.wfp.org/what-is-blockchain-and-how-is-it-connected-to-fighting-hunger-7f1b42da9fe https://www.reddit.com/r/ethtrader/comments/6144xe/ill_just_leave_this_here/dffpbf1/Also there are hundreds of dapps in work, beta, or already deployed on ethereum. There are just too many examples, you'd have to search for them yourself.
Ethereum is the only decentralized blockchain project that aims to solve scalability, both off chain with raiden and on chain with the implementation of casper. No other project has a team competent enough to tackle onchain scaling ( they do flirt with offchain scaling on bitcoin's side, something similar to raiden ).
Ethereum is backed by fortune 500 corporations and institutions, which are working closely with the ethereum foundation ( microsoft, jpmorgan, santander, thomson reuters, united nations, etc )
Ethereum is the most advanced dapp/smart contract platform. Any other project is a joke in comparison.
There are hundreds of developers not afiliated with the Ethereum foundation that, one way or another, work on ethereum.
There are many more advantages but i can't either remember them all, or it's just because i'm not technical. You'd have to do more research yourself.
6. One disadvantage now is that ETH is still POW and just like bitcoin, it can be 51% attacked, altough it would still cost tens of milions. But every other project, excepting bitcoin, is even more vulnerable because of the smaller security.
Another disadvantage is that it doesn't have yet privacy while some projects do, but that will be fixed in ~2months.
One more would be the fact that it doesn't scale yet, but no other project scales ( by scaling i mean close to visa level, or better ). Raiden will bring offchain scaling, and 1mil + transactions per second will be possible.
Another disadvantage is that, because eth is a decentralized platform, and since everyone can deploy projects on it, some of the random amateur developing teams out there may not be competent enough and may deploy projects that are vulnerable to attacks or simply broken.
7. How can it help develop the blockchain ecosystem ? Read all above. Also bitcoin mostly gained the attention of random joes, ethereum gained the attention of people who matter, people that have funds, decision power or political influence.
8. Depends what you mean by mainstream. As a global payment system, with raiden, ethereum could handle it, since it would scale more than visa. As a dapp platform, i think ( not sure ! ) it still needs to scale onchain with casper next year.
