There is no back-focus at infinity.
The lens' infinity stop is set for image distance = focal length, and most Leica lenses have the exact focal length specified on them.
Everything else should be adjusted to center accuracy and wide open. Now, with 20+ year old lenses from ebay, there is a large chance that a used lens / camera was adjusted to a shooter's preferences for close focus. Just check out how often people here talk about CLA this, CLA that. Over the last 10 years or so, the used lenses that I've seen easily have variied +- 2inches wide open, at 1m focus.
You can either adjust all your lenses to your preference, or make sure the lens behaves as it should at infinity (wide open center resolution), and "learn" how each lens behaves at close focus.
At close focus, I personally prefer - in particular for moderate wides - a slight front focus (1-2 inches, depending on focal length), to compensate for cosine errors at 1/3rd composition, and make the lens sharper when it shifts closing down 1 or 2 stops (all of your 3 lenses shift backwards noticeably). Other users will be different.
In a nutshell, at the accuracy levels that you are talking about, Leica lens compatibility is a myth. With used and even with new lenses, in particular if you add other brands (Minolta, CV, Canon, Nikkor, etc).