Most of the structural mass is devoted to bulkheads and supporting elements. 3D printing can help with reducing the mass of non structural components too.
Bulkheads may have relatively simple shapes, but that's in part because the milling process imposes limitations on the kinds of geometries available. I'm aware that sintered material has less strength than casted ones, but the strength of a structure isn't just determined by material, but also how shape distributes force loads. That point on shaping doesn't have to refer to anything exotic like more curves either. It can refer to something as simple as dimensional limitations imposed by the casting and milling process. In areas where the expected force load is light you can afford to shave off more mass up to a certain point, but there's a limitation to that when you have to cast then mill the titanium (refer to the picture of the sintered part in Engineer's example).
As Engineer suggested, sintering also allows you to eliminate places where you'd have to weld the material, which means to get the same amount of strength in an area with welded parts you can afford lighter structures. This point is particularly true of support pieces that require welding, as welds can be where some of the greatest force loads are concentrated. Even though the crystalline structure of sintered pieces are imperfect a consistent shape will have fewer structural discontinuities and points of failure, which allows for the transference of force loads to where they're supposed to go and make the overall structure stronger. Finally, I'll let your own picture of the F-22 bulkhead answer your point about bulkhead shaping with regards to holes, gaps, curves, and right angles.
Your deduction doesn't necessarily conclude that those planes can't supercruise without a F119 class engine, but simply that Russia didn't think those engines fit their performance requirements. It's not like all supercruising is equal. I think the primary contention over this point has been whether the J-20 can supercruise with the AL-31, not whether that supercruising performance is adequate.
