Cathedral ceiling retrofit with no ridge beam or ridge board - what are my options?
Dan, the horizontal slats, resting on the roof joists, replaced your ridge board (not the best thing to do but it is an alternative).  Worst case scenario: someone removes these when replacing roof decking or to install a skylight.

If your plan is to remove/block-off any of the attic, up to the roof deck, you'll need to replace the gable vent with the combination of soffit-ridge venting (they work together).

Your drawing describes the installation of a ridge beam.
Take a look at this: ridge beam vs ridge board

Ridge vents are sized to the space they vent.  They work differently (or not well) if one uses them in conjunction with soffit vents or ridge vents.

Re: Knob &amp; Tube Wiring.  Assume you have K&amp;T throughout the house (in the walls). Many careless/short-cut re-wiring jobs are made by keeping the K&amp;T within the walls but re-wiring just the switches/outlets to make it appear all the K&amp;T was removed.

All-in-all, you should find someone with old house restoration experience.  Such people are most qualified to understand what you have and how to make it what you want.  General remodeling contractors and the usual, run-of-the-mill architects tend not to know such things.