Star alliance doesn't have seat selection as a *Gold benefit - it'll be whatever LH provides for your ticket (and if they extend any benefits to *G like Air NewZealand)

You get your 1K extra allowance when checking in with UA (typically 3x70lb, you need to verify your route on UA baggage calculator)

On the way back, you check in with LH, and only get *Gold extra allowance (usually one extra bag at 50lb, again you need to verify your route on LH website)