Thank you for your questions!
Putting the cords beside the lamms allows the lamms to pass each other when you press on a treadle. If the cord goes through both upper and lower lamms then they are more likely to hit one another when you try to open the shed wide because they are essentially tied together. This mostly happens when you are using more treadles, but when it happens, you can feel the resistance.
I have seen people move the beater back as they are weaving. I prefer advancing the weaving rather than moving the beater. It allows me to get up and look at my weaving (I often find my weaving errors when I look at my piece from the side). Depending on how large your loom is, the beater can be kind of heavy and cumbersome to move.
Moving your beater back changes the pivot point of your beater swing. Sometimes that can effect how hard or the angle that the reed is hitting the fell line. That may be what is causing streaks. For me, often my streaks are because I don't advance my weaving enough. I am always pushing, by either over advancing or weaving beyond where I should. Meaning that I make it harder on myself to keep a consistent beat. Moral is, advance often, this will keep you closer to the weaving sweet spot and help keep your beat more consistent.
I believe the best use of the grooves in the beater cradle is when we are trying to use the last of the warp and as an adjustment if you have a longer reach and need the beater further from you to be comfortable when weaving.
Hope this helps,