Electrum's own site states that the wallet "was created by Thomas Voegtlin in November 2011" and lists him as its lead developer. The repository is consistent with that: the earliest commits on master date to December 2011, and the project metadata records him as the original author.
The same site names the entity: "Electrum Technologies was founded by Thomas Voegtlin in 2013", based in 10999 Berlin, Germany, and it is Electrum Technologies GmbH that distributes the MIT-licensed software.
The design decision that matters is simplified payment verification. Rather than downloading the whole block chain, Electrum fetches block headers and checks its transaction history against them, which in 2011 was the difference between a wallet you could install in a minute and one you could install in a week. The tradeoff, that an indexing server sees which addresses you asked about, is answered by letting users run their own server.