BigL, this is what I did on my 2nd cycle and it worked pretty good. For the 15s, I did 1 set of each compound and did not do my isolations at all. During the 10s, I did 2 sets of compounds, 1 set of each isolation. Then I did the same during the 5s, except I would try to do a third set for each compound if I felt up to it.
It worked really well.
It is true that you do not want to hit failure on the second or third set. What you can do if you get close to failure is just terminate the set then. You could even go to clustering if you really wanted to. Just don't go to failure.
As for SD - yes it is hard, but remember that the purpose is to decondition your muscles so they will respond to the lighter weights. So basically the longer you SD, the better your muscles will respond. Is that incentive enough? It sounds strange, but that extra week of SD that you want to skip can actually make a significant impact on your gains.