1. Is it not possible to create an Exchange, based on cross-chain Integration { Not between BTC to ETH only but Between top 100 Projects } where a user can buy and sell tokens by directly dealing with the blockchain and trade with all of the buyers and sellers who placing orders on that?
Aside from LN, it's possible if each coin (or coin used by the token) have feature (e.g. script and smart contract) which can be used to implement atomic swap. With atomic swap/LN support, all you need is platform to host the order book.
5. Serum is known as the first DEX but they make it a little complicated for normal users, Is it not possible to provide easy access or a simple login feature over decentralized exchanges?
The UI/UX of the software/website could be improved, but since we're talking about DEX, concept such as login is impossible.