If you are worried about the bending on the two rear racks... get a hunk of square aluminum tubing and bolt it to the bottom of both tracks across the joint. You should be able to find a carriage bolt that will fit the channel on the bottom of the racks and bolt to the tubing. Probably about a 5/16" I would say.
The side to side is controlled entirely by the front fork mounting. Some give will not hurt anything unless it begins to oscillate. If you need more there, add another cross bar tied on each side/end of the factory bar and to the bike rack head.
I have a Tandem Topper and A) it wobbles side to side a bit and B) there is some wiggle to the rear section. So it sounds like you are as good as the real stuff.