The best way to mount a rear rack I've bought Bontrager Backrack light and I am thinking about different ways to mount it on my road bike.

Some details:


at the bottom the rack attaches to the quick release screw.
the tubes connecting the rack to the frame at the top are attached to the rack by a clamp as shown below. The clamp sits on top of a racks tube and the connecting tube is inserted through the holes. With tightening the screw we push the tubes together.
this clamp can be mounted on only one transversal tube, others are too wide.




I can attach the rack to the brake mount adapter or to a special seatpost clamp.

Questions 1 and 2

I prefer the seatpost clamp (pictures S1 and S2) since the brake mount adapter doesn't seem strong (I can rock the rack and bend the adapter, pictures B1 and B2).
Is there any downside to the seatpost clamp mounting, e.g. putting too much stress on that part of the frame?
Also, is the high angle of the tubes problematic?
The tube is only in an approximate position since I haven't bought the appropriate seatpost clamp.
The tube behind was left there from the brake adapter mounting.

Question 3: 

Which orientation of the connecting tube is the best?
Obviously, I will cut the tubes to the correct length when done.
There is another option that is preferable to either of the ones you mention: P-clamps. These are small strips of sheetmetal bent into a P shape and covered with rubber. They fit around the seatstays to create mounting points. You'll be able to attach both of the top rack arms that way. This will be much more secure. P-clamps are cheap and you can probably find them at any decent bike shop or online store.

Once you've got P-clamps on, I would mount the top rack arms as pictured in B1, more or less, but that's mostly a cosmetic thing. B2 wouldn't really be different in terms of load transmission.