I opened up the sash balance mechanism by grinding the staked keepers that hold the cover in place. Then inserting a putty knife into the housing (had to bend the end cover up for access) and over the spools (to protect them), then a flat screwdriver on top of that knife and turning it to pry the cover free. Not much pressure is required, if you have ground enough of the staking away. I was then able to replace the broken cord with some nylon masons line. I wound the new line around the drum and used it to rewind the spring, repeating until it looked like the other one that was not broken. I then used a small c-clamp to keep it wound and proceeded to place the correct number of wraps of lines on the drum to duplicate the unbroken one (total of 3 ½ turns). Then threaded the line out through the hole in the housing and through the plastic pellet and tied a big knot in it. Then released the c-clamp, while holding the spool with my thumb, and slowly allowing the tension to pull the plastic pellet into place against the housing hole. (I wore work gloves during the rewinding operation so I could safely hold the spool with my thumb when needing to do subsequent rewinding on it. From past experience replacing tape measure tapes, I was careful to watch for and avoid the spring going ballistic on me. It never really threatened to though.) Then I replaced the cover so that the ground away posts were in the opening where they belonged and taped the cover on with USPS packaging tape. :) The cover isn't in danger of coming off since it's held in compression when in the installed position. I wish I had been able to find some heavier line but I think the masons line will likely last well enough. It's rated at 155-pound test. If it doesn't, there is a heavier masons line available that is 216-pound test that I'll replace it with. A tip getting that left side track back into place without breaking it or hurting yourself is to slather some liquid dish soap onto the window stud. It slid right into place easily after I resorted to that method. It also helps to file the sharp end of the spline on the back of the track so it doesn't gouge into the stud. It has taken me nearly as long to post this than to replace the broken sash cord line. I hope it helps someone like me who couldn't find any info on doing this and had to stumble through it. I like fixing things rather than tossing them, especially when it only needs a piece of string replaced. YMMV

I also use Goof-Off to work out a scratch. Put it on one of those green scouring pads used in the kitchen. Work the scratch until you see it starting to disappear. Don't use a lot of Acetone or Goof-Off. You will see the vinyl start to melt down and fill the scratch. Ounce you've taken it down to where it's almost gone you can flip the green pad to a dry area and buff it up to that shine you want to see. This also works on any vinyl window. Just be very careful!!!!

Thanks guys! I should have mentioned we tried the epoxy idea before. I even used a rolling pin to press it flat. The trouble was that the clamps weren't really long enough. We put a piece of wood on it but the clamps just aren't deep enough to fully press out that bubble. It lasted for one winter. Guess it's time for a new door... sigh.

I would skip the Marvin Infinity and go for the Marvin Elevate if you want a non-vinyl option. If you aren't against vinyl (Fibrex is 60% vinyl by constitution), then I would look at one of the higher end vinyl replacement windows. They will offer better thermal and air data (i.e. performance) as well as stand up fine to hail.