Hey there!
The 15-111 looks like a match. This pivot shoe is used with the 1/2" S160 coil springs (sold separately). I recommend checking the tongue of the spring for a stamped number. If there aren't any, you must weigh the window sash and divide its weight by the total number of springs in the window.
I'll be here if you have any questions!
You only have to replace what's broken so you can reuse the old parts if they're in good shape. We don't have a 4-pound coil spring but you can use one S160-03 3.5-pound and one S160-04 4.5-pound spring on each side of the window. 8 pounds of support on each side will total 16 pounds of overall support.
I'm not sure what you mean. We don't have a version of the S160 with a longer tongue. Our parts should still work for you though.
That principle doesn't apply for the Series 160 springs so they'll work for you as is. The spring band is 1/2" wide so you'll want to make sure yours is as well.