Yeah, I'm afraid it's gotta be option #1 (modified). Because, I've carefully checked the position of the subfloor girder relative to the existing header (and its plumb supports) and the supports actually are completely off to the side of the girder. Sorry stuff - monstrous shear forces on the subfloor plywood ! So by sistering LVL to the kitchen side of the ex header (cutting off the ends of the ceiling joists, and re-attaching with joist hangers), I get it, and the new support walls, more or less centered over the girder.
I'm thinking I can solve the support problem (the fact that half the total big gable rafter load could be coming down at a point, where the ex header is spliced, near the center of the LVL); the LVL can take it, but how to make the connection ? Instead, I'll take the big gable rafter loads off the ex header. I'll pound joist hangers underneath the edges of the rafters, right where they rest on the ex header (do a little Sawzall'ing if needed), so the face of the joist hanger (that nails to a beam) is flush with the kitchen side of the ex header. Then sister a 14" wide LVL beam to the kitchen side of the ex header, reattach the cut-off kitchen ceiling joists, and nail the joist hangers that I put under the rafters to the other face of the 14". Maybe jack up the ex header a tiny fraction of an inch, so the rafter loads come down on the joist hangers instead of the ex header. A single 14" LVL can easily take the load, and the ex header should provide good lateral bracing; I might decide to use two 14" LVLs though; centers things over the girder better.
They'll be small joist hangers (since the 14" will stick up only 4" or so above the ex header), but right now, those rafters are simply sitting on top of the ex header, toe-nailed with no other bracing (except the roof decking 1/2" OSB). So a 2x4-sized joist hanger will be an improvement. And I can nail a 2x4 along the tops of the rafter ends, to provide some better bracing.
