How is the rafter bearing on the top plate of the wall without a seat cut (birds mouth)?
The simplest solution from what you said are looking to do is to
1)collar ties go in the top 1/3 of the triangle. They prevent the rafters from being pushed apart at the ridge during wind events. You can you metal coil ridge strapping on the roof if you plan on reroofing in place of collar ties. R800 in the IRC has a description of that the size of the collar ties should be. (No less than nominal 1x4)
2) Install new ceiling joists 1/3 up the overall height of the roof measured up from the top plate of the wall. Install these first and nail them off according to the IRC chart in the R600 section of the code table R603.2(1)
3) Once the new ceiling joists are in place in theory you should be able to cut out the old ceiling joists.
My understanding on your project is
A) The ceiling joists and rafters run past the wall framing to form the eave/soffit framing on your home, correct?
B) The rafters lack bird mouths
you can probably use these to remediate the lack of birds mouth
I would probably pull the soffit down outside and try to gusset the rafter and ceiling joist in the eave together with some osb or plywood. You may need to install some 2x furring to pack the joist or rafter out with the other since they should be lapping next to each other. Then cut and fit a plywood gusset to the rafter and joist to make a solid triangle.
I'm personally not sure if this is worth whatever height gain you'll be getting but that is for you to decide.
I haven't thoroughly read through the rest of the thread so maybe I'm missing other important details.