Hi Thomas,
Timber framing is my primary wheelhouse, but many different forms of traditional cobb and adobe building are in my background. To
answer you first question, slip, whether you are dealing with ceramics or cobb, is nothing more than the "clay body," you are working in, thinned to a consistency that you want, (i.e. thin as paint or thick as cake frosting.) So don't worry to much. Painting on is the least common method I have seen, most often (if used at all) it is just spread on like frosting between the "lifts" of work you do on your wall. When I personally do "mud work," I like to keep the wall area I am adding to moist, and if there is going to be a "cold joint," (i.e. where you stop working is a cold joint) I will insert (about 100mm to 150mm deep) small (12mm to 20mm diameter) sections of
wood or bamboo every 150mm to 300mm apart, leaving proud about 100mm to 150mm, to take the next lift of cobb. Then the first
course to go on is really 'wet' and 'squishes' out when I add consecutive material. That is my form of a cold joint with mechanical bonding as well as 'slip mortar.' Note this method is a culmination of several traditional methods including Dine' and Pueblo.
There are just so many different styles and approach methodologies, that you just find degrees of strength. The method I described above is for structural walls that could go as high as several stories. I would have to know more about your build to really get "picky." Sounds like you are doing good work and really making an effort to build well.
Regards,
jay