I'm working on a project requiring a larger multi-touch monitor, and I'm using a 23" Acer T232HL, but Raspbian seems to be treating it as just a mouse. In other words, no touchscreen conventions like long-tapping, and no recognition of multiple touches; it just registers a left-click at the location...

No. Steam In Home Streaming requires the proprietary Steam client, which does not, and probably never will, have a version that can run on any version of the Pi; Limelight is the closes the Pi will ever get. That said, they just announced a $50 device called the Steam Link, coming out in November, t...