a) disable mask, paste (new layer will be added), merge down created layer, reenable the
You can also enable mask for created layer, so base layer will have 2 masks. If layer has multiple masks they are first virtually merged like normal layers, then used to calculate pixels.
b) sorry, there is not „paint selection” tool yet. Workaround - create empty layer, paint area with Paint tool, then tap Select>Alpha To Selection>New
