#2) Yes , just a smoothing layer to direct the hot air to your pipes.
#3) If you can turn that cleanout it would be more convenient. If you can't, you can work with what you have. If your rocket is built to spec and burning cleanly (good dry wood) you will not produce enough fly ash to go more than a few feet down your horizontal. You have your cleanout at the manifold , the one at the 180, and your vertical leaving the mass is able to lift off. It is enough. Mine has been burning for 5 years , I have one cleanout at the manifold I have none at my 180... (i really should have put one there) My vertical pipe IS able to lift off for emergency cleaning.... I have not yet needed to lift it. At one point my stove was not built to spec... seemed to burn good until middle of winter when it just wasn't rockety anymore. Turns out I was not burning cleanly and produced enough ash to partially plug my horizontal. Vacuuming cleaned up the visible run ... that was when I learned that vacuums do not like fly ash... I also learned that using a leaf blower works much better, as long as you remember to remove the chimney cap first... ha ha
#4) I wondered the same thing when I built. So as I came up with the cob I placed large flat rock over top of everything to distribute the weight over a larger area. The cob itself is very strong when dry.
Last of all ) I recommend you watch the video just posted by Matt Walker about mortar making it is very informative!