Bill,
My guess is that a residual stress was introduced when you replaced the brace after the gear failure. This has been acting on the windshield ever since, and along with operational loads, has given you this new crack.
Drilling the crack tip will reduce the stress concentration from infinity to some calculable value. This will make things a whole lot better for awhile, however, it won't necessarily prevent the crack continuing to propagate. Gluing carbon fiber across the crack MAY restore the structural integrity, but it won't remove the residual strain at the crack tip. Unfortunately, the initial crack probably transformed a rather broadly curved distortion in the windshield opening into a much more angular one. Adding the carbon fiber reinforcement will reinforce the deformation rather than remove it. Until the deformation is removed or the crack propagates far enough to reduce the strain to an acceptable level, the crack will keep growing.
It is possible that the crack has already extended far enough to reduce the strain and that drilling the crack tip will arrest the problem, but I doubt it. It will probably continue to propagate some additional distance before the strain is adequately reduced and drilling the crack will be effective.
I can envision braces being attached to the windshield opening to restore it to its original shape, and thereby removing the residual stress in the plexiglass. You might also be able to reinforce the windshield in front of the crack tip to prevent the distortion from traveling with the crack, but this will probably impair vision. All of this seems a lot more complicated and involved than just cutting out the windshield and gluing in a replacement.
Sorry this isn't what you wanted to hear.
Rob