Correct, & I should probably have been clearer about it.
Um, yaaaas. Even now your reasoning in comparing a pair of central engines driving a pusher contraprop to a trimotor can only be guessed at.
(The XB-42 page has the pic, & the piston DC-8 page doesn't...but there is a link to it.)
The link isn't actually there, you know. I had to go via
Mid-engined aircraft to
Twin-engined single-prop pusher aircraft to finally find
Douglas DC-8 (piston airliner). Your mind is, shall we say...subtle and indirect?
Speculating, I suppose you suggest the piston DC-8 would have been relevant in that one virtue of a trimotor is that if one of the three engines goes out, the force imbalance of the remaining two is either zero (if it is the central one that fails) or not so bad, as one of the two remaining is the central one. This is all the more true of a cluster of internal engines driving a single central prop arrangement of course!
The trouble with a central pusher prop is that it is mounted on the tail, and thus way behind the center of mass, so when the airplane is trying to take off (or land) its angle of attack is limited by trying to avoid grinding the prop into the ground.
Another good trick a trimotor might try is shutting down one or two engines in flight to save fuel, extend endurance and possibly range (not certainly, if the loss of cruising speed is greater than the extended endurance--but I think the math for cruising speed works out to favor range as well). It depends on how much one loses to drag from the shut-down props how good an idea this is, and of course whether the airplane can stay airborne on 2/3 or even 1/3 power. I've seen pictures of trimotor designs where the two outboard engines are replaced with more powerful ones and the central engine is deleted--then of course we no longer have a trimotor.
If you want a trimotor with shaft-driven offset props, look at
these monsters! Three engines in a forward bay, driving two props off to the side by a drive shaft to each.
I'm flabbergasted; I gather it is quite difficult to synchronize two piston engines to drive one shaft and here there are three engines.

Apparently it worked, but if you want to have the ability to shut down one or more, well that would work best with variable pitch props which are definitely not available in a WWI context.
This picture gives the clearest idea of the arrangement... the scale is best shown
here.
None of this seems to address the OP's concerns very directly though.
