Regarding the "solver failure" a couple of pages of posts ago, I found it strange. In my experience, end-point-to-end-point tangency works quite nicely.
If there is still interest that I look into it, if you can save the sketch just in the moment before adding the offending arc, you could open a ticket with it. I will look into it in August-September.
Replying to your question. I am not aware of any way of forcing the solver to comply with ones undeclared wishes. The only way that generally works is declaring the wishes by providing a solution as close as possible before actually asking the solver to solve. The solver, as most people, is lazy enough to choose the easiest way out