IKO organic shingles, eh? Have there been any recalls? Complaints? Why organic instead of fiberglass?
You don't need continuous soffit venting....just soffit venting period. In solid wood, you can drill holes sized for round air vents or cut squares/rectangles if room permits. ANY lower venting is better than zero.
Camera optics may explain but it looks to me like your roof sags from left to right. Further, it looks like the damage is worse on the left side, than the right. In an older home, I'm guessing it was stick framed. So the rafters might require some additional bracing.
I had 33yr old cheap 3-tabs on a 5/12 pitch, in Texas heat, that didn't look nearly as bad as your 15's. We don't have ice down here though. You have potato chips.
Due to its Southern exposure and pitch, your lower vents won't work as intakes. Did your neighbor have this problem prior to replacement? How does your decking look from inside the attic? Is it ply or chip-board? I'm guessing you have an attic moisture problem could be due to a roof leak or from humid living area air rising up into the attic, then condensing on a cooler night-time surface.
Good luck.