Launch rail (rod) and backplate issues (reprise)
Re making the Teflon insert slightly larger than the diameter of the launch rod: this might actually make things worse for two reasons.
The first reason, and one that applies more at lower altitudes is that by leaving a slight gap between the insert and the rod you're leaving space for the ingress of wind-blown dust between the insert and the rod and once the dust is in there it won't get blown out again.
"Wind-blown dust?", I hear you say. Yes, the stuff that water droplets, which we see as clouds, nucleate around, which brings us to the more serious problem with leaving a gap between the insert and the rod and which applies more at higher altitudes.
I assume that you're going to launch on a cloudless day, so you can at least watch LOHAN for a good part of its journey, and lack of clouds will imply low dust levels: all well and good. However, this doesn't necessarily mean that there won't be much moisture in the upper atmosphere; it just might mean that there's nothing for the moisture to nucleate around/condense upon. The trouble is that when LOHAN drifts up through this air then the water will be able to condense upon it and in this case, contrary to the first mental image that may be conjured up, a moist LOHAN is not going to be a good thing.
With a relatively long launch rod, and assuming that the moisture isn't already super-cooled, any moisture that condenses upon the rod will run down it and into the slight gap between the insert and the rod and once it's there capillary action will keep it it there, almost guaranteeing freezing between the rod and the insert as LOHAN rises even further (if the moisture is already super-cooled then it'll freeze as soon as it condenses). Either of these conditions could be tricky, if they actually occur and the longer the launch rod, the worse the problems would be.
One possible solution is to (once again) do away with the single launch rod and instead use two 'L' shaped launch rods hanging down from the truss. You would then use a much shorter 'running length' than currently planned, let's say about 4-6 inches in old money, but then you'd be able to completely enclose them at the front, preventing any water ingress. Obviously, the vertical length of the two 'L' shaped hanger would have to be unequal, with the rear hanger being longer than the front so that the rear attachment point on the aircraft clears the front hanger.
Re the backplate: Whilst making it thinner/narrower is a good idea, which will reduce the back-force acting upon it, if it's going to support a long cantilevered launch rod it'll still have to be quite substantial. Going for the two 'L' shaped launch hangers would remove the need for it entirely though, as you could then put the 'stop' on one of the hanger runners, within the covering shroud, or even extend one of the covering shrouds backwards to act as a stop upon one of the hanger uprights.