Is it the steel frame work and coupler section, or the area that makes up the aluminum gooseneck/bunk area??

Stress cracks are an indictaion of structural problems, not usually weld issues. By rewelding the joint, not only will the hardness and strength of the aluminum be adversely affected, but the underlying problem would not have been addressed. A reweld could actually make the matter worse.

As Hosspuller mentioned, aircraft too suffer stress cracks. They can be found on fuselage mounted engines near the attach fittings, openings such as door frames, or near pressure bulkheads. They are not repaired by welding.

Engineers of the manufacturer and the FAA, figure out how much additional material is necessary to add to the structure, to prevent it from failing under the applicable stresses. Additional pieces of structures are mechanically fastened in these areas. A second strengthening piece would be a "doubler". In some instances, a third or fourth piece can be added. They are affixed by blind or solid rivets in some instances, bolts in others.

The only fix that would provide a permant repair, would be the installation of additional materials. Done properly, their size and shape would involve some engineering and imagineering. The present structure is apparently insufficient for its weight.

You did a good job with your inspection. Many people would not have been so thorough, and likely would have missed the problems.

As Hosspuller also said, "pass it by". Good advice.