This has become the new row style. It's what everyone is doing. JS rows have just created a monster of bad rowing. I can tell you exactly what is going wrong but in order to sort is out you'd have to drop the weight and do a ligher one not off the floor. Or a cable row would be good to nail down basic technique.
What you are doing not will eventually comprimise your shoulders and thus upper back. You lower back has not much of a set in it. I won't say more until you respond except that there is basic technique flaw that is being compounded by the weight over quality aspect of this lift.
Somewhere along the line the "internet row" was invented. The row has become a primary lift instead of an assistance so people do the same crazy shit they do on squats, deadlifts, bench, whatever to get as much weight as possible. I still haven't seen any row contests but I'm sure someone will invent a "row shirt"
As I've said to everyone doing this....why not just do cleans or something if you want to pull something so fast and heavy instead of doing the bastard chilld of a row, SLDL and clean, etc...
The pullthroughs are exactly what you said. I don't know if you need to change postion. That's something you have to play around with until you lock in the position that feels right. You kinda just walked out and went to town without any adjustment. I know for myself I have to move around to find the sweet spot everytime the weights go up.
But what you need to do is just SLOW DOWN. Pullthroughs are not a power lift. You need th negative to be slow enough that you can concentrate on keeping the back set and dong hip flexion. You are simply letting the wieghts pull you back and down (probably having to do with position also). Then you are just yanking it up. Like you said though your werent paying attention to form so I know you are already aware of this. You will probably be able to go heavier once you get it down.
I could be imagining this but it seems like your knees are a bit on the inside...not completely in line with you feel. Also it seems that your feet are spilling over the soles of you shoes...a bad situation.