First question: Can someone trace the 0.5 XMR back to my Monero GUI Wallet?
No it cannot be traced. Monero runs on a blockchain that is privacy oriented and the transaction is said to be private through the use of shealth addresses. If a transaction isn’t visible then it will be hard to trace the transaction back to where it originated from and such can not link addresses together.
Second question: Is LocalMonero a mixer/tumbler for XMR coins? If your answer is "yes", I assume that AgoraDesk is also a mixer/tumber for BTC coins?
No both the localMonero and AgoraDesk are decentralized P2P trading exchanges where you convert your coins to fiat or vice versa without having to submit KYC. They are not in any way a mixer or tumbler. You. Can visit the [ur=https://bitcointalk.org/index.php?topic=5188930.0]AgoraDesk[/url] ANN thread.
P.S.: I searched everywhere for answers to the above questions on the whole of this forum and found none; hence this post.
Here is the thread designated for XMR on the forum
https://bitcointalk.org/index.php?topic=583449.0