You started a new discussion piggyback onto a nearly 10-year-old general discussion "horizontal shaft having asymmetrical stiffness" that you apparently believe is in some way related to the cause of your specific problem. It would’ve been far better to read your discussion as separate posting. If I understand correctly, your hypothesis is that the shaft coupling between the compressor rotor and the turbine rotor has asymmetric stiffness, and that is the cause of the 2x vibration response.
The coupling is a rigid joint between two shafts of symmetrical stiffness; as an assumption. The coupling is of symmetrical design, but it may be assembled in a way that causes asymmetric stiffness; as an assumption. The hypothesis of asymmetric stiffness can be tested with the rotors removed from the case and hanging from slings or resting on soft supports at the bearing journals. Impulse-response tests should be conducted on each rotor while uncoupled. This would identify rotor bending natural frequencies; that would be different from the balance resonance frequencies with the rotor on the bearings and in operation. The rotors would then be coupled so that the composite natural frequencies can be measured with the shaft in several index/angular positions while keeping the impulse-response measurements in the horizontal plane. If the coupling is acting as an asymmetric stiffness joint between the two shafts, then there should be a significant difference between the natural frequencies in orthogonal directions. If this is the case, then try reassembling the coupling a few times to see how consistent the results are. This test is obviously limited to evaluating coupling stiffness under cold nonoperating conditions. The test does not indicate what would happen to the coupling stiffness as the shaft thrust load and temperature change. It would be difficult, perhaps not impossible, to apply an axial static load and measure the change in rotor bending natural frequencies. I would be considering other possible causes to the high vibration before pursuing this hypothesis and tests.
My hypothesis for the information that you provided is that the dynamic force is from shaft misalignment at that either a shaft critical speed or stationary structural resonance (most likely) is excited at a very narrow speed range. My hypothesis assumes the opening of the bypass valve to be coincidental with the onset of the high vibration. I realize it was stated that the shaft alignment was okay, but this does not necessarily indicate that the tolerance was adequate (by procedure or adequacy of results) or that thermal distortion of the turbine casing could change shaft alignment during operation. It should be easy to conduct an impulse-response test on the stationary structure including bearing housings, compressor, turbine, and exhaust casings to look for a natural frequency near 435 Hz.
If a thorough test of the stationary structure does not reveal a natural frequency, then it should be assumed that it is a rotor natural frequency that is being excited. There is some evidence that this is possible, since compressor blade tip rubbing was reported.
I missed or misread your bearing description, so sorry for that. I did find it interesting and coincidental that the power turbine critical speed was nearly identically to the frequency of high vibration. The power turbine shaft speed is not synchronous with the gas turbine shaft speed, so it would be a big coincidence if the speeds were the same at 87% speed. I did not see mention of the PT rotor speed, but I assume it is being measured.
You have with reached one clear result from disassembling and reassembling the machine; the same parts and same assembly methods do not change vibration nor lead to an understanding of what is causing it!