I guess your assumptions what's going on are not completely accurate
What i tried to show playing with the commandline was just that a completely wright cl gives totally weird error.
And a totally wrong cl results in a message saying ".. contains invalid path" even though all path's displayed are perfectly fine.
Ok but i agree - this leads to totally nothing and was just experimental
As i found out in the meantime there are huge code parts in the game here.
In short: UE checks for another running instance if found calls GlobalAddAtom() and sends (an already wrong) cl to it's instance.
U can easily check this out with something like:
hWin = WinFindM("UltraEdit");
hAtom = GlobalAddAtom("/s=yourclhere");
SendMessage(hWin, 0xC144, 0, hAtom);
This sends a correct cl from your app -> resulting in the same strange Message -> bad luck
I also found out, that the receiving end in UE then messes it up completely, probably not recognizing it as a cl-option, instead replaces '/' with '\' (thinks option is a file) ..
OK - again - this leads to nada-land.
But the nice thing is, by fooling around with this i found a (already working) workaround.
I simply register my ufccopy.js (or what else) with a hotkey in my case ctl-alt-F12 (never hit by user).
When i now send a hot-key simulation message from my app to UE via:
SendMessage(hWinUE, WM_NOTIFY(0x111), 0x0001FF78, 0)
voila - all the mimicry is in action - I'm now able to call and execute ufccopy.js on a running instance !
Maybe this can help others if in need