We usually dough-in with a .75 ratio just to saturate the grains. Then we infuse and bring the ratio to 1.25 for a thick mash. What determines the ratio is you. As long as you are between 1 and 2, you'll be fine. Over time you'll find the ratio that works best for your recipes / brewery.
Warm water goes into the tun and then the grain is added and stirred into the water. We pour some grain in, stir a bit, pour some more, and stir until it all get in there and is wet.
I wouldn't put glass into a boiling kettle. Ingesting a glass chip is a horrible experience, so I would be somewhat concerned about that. We don't put anything in the kettle to control boilovers, however, we always have one hand on the gas flow valve for the burner. Dropping the flame on the kettle manages boilover as do several other methods.