a) How does the logic behind it look like? (see lögic sketch)
More or less, yes. Project requirements are usually not stored on the blockchain/smart contract. They are off-chain (on the website database or even on IPFS). Only the funding and token generation logic are on-chain.
b) Would it still be worth to utilize some of the blockchain features (voting rights, transaction fees, transparency), if 'Great Coin' would centralize the approval for the 'Super Crafty Smart Guys Project' before it presents the 'Super Smart Project' to the 'Great Coin Community'?
Depending on the platform. Some platforms have a centralized authority that approves the projects, others let the community to decide through a voting mechanism and others accept everything (usually after a payment).
There are different projects that include those concepts of voting rights, transparency, etc. to improve the funding. Look for "Coin Governance System", "DAICO", etc.
