1 - I haven't built a baritone electric, but I have built a very long scale 12 string (26.5) that is designed for tuning C to C. I would spend some time on the D'Addario website calculating string tensions for the scale and tuning you want to use - in order to maintain playability on my 12 I have to bump the string gauges up considerably. I also entered you scale length into the SM fret position calculator and it will give you the location of each of their standard electric bridges. Make sure that you have enough travel in your bridge saddles to get proper intonation.
http://www.stewmac.com/FretCalculator
2 - I use exclusively scarf joints now (this winter alone I have repaired four guitars with broken headstocks including a lovely old Gibson J-35). I simply take a piece of 1" thick mahogany (do your laminate or whatever) and cut it off at the correct angle (usually 16 degrees). Flip it over and glue up - you can either put the scarf at the end of the neck or under the fretboard - I like the latter. I also stack the heel out of one inch blocks. Jig this up carefully - the pieces want to skate around on the wet glue
On my acoustics I put the truss rod adjuster inside the box, but on electrics I like to use a double acting rod with the smallest possible adjuster and make that route as small as possible (that is where all the Gibsons seem to break)
AFAIK Gibson, like Martin, uses a one piece neck and cuts the angle. That seems wasteful of materials and creates a weak area where the grain is at an angle. The truss rod route just further weakens it. I know that Taylor is now using a scarf joint (they used to use a finger joint) - don't know who else is doing it this way.