RPI WiFI is a small chip on a low-powered board with no antennas. This situation is not helped by adding HATs and/or cases. There are a number of USB-WiFi adapters that people reported success with. I use a last-gen Airport Express as bridge to a Cubox – works flawlessly, even in high bitrates.
For 2. you’ll want to downmix your output signal to mono – you can do that using Procedural EQ in Roon. This explains how: