How to display Flow Direction arrows + Burn Stream Netwok in QGIS?

Regarding your first question (it's usually better to focus on one question per post):

In GRASS you can display vectors with direction arrows. In the gui there's a checkbox "Display direction of linear features".

So if you have run r.watershed, then you probably have created a raster stream network. With two addition commands you can convert this to a vector map of streams then display with direction arrows.

First do r.thin to make sure that the raster stream network is thinned to only a width of only one cell all along the streams. Then run ... type=line to convert to vector. The resulting vector will automatically have the line directions in the direction of flow.