I think "decreasing the slant" and if possible, raising the roof overall, could be a doable plan - it would 1) allow you to add much needed ventilation over their heads and 2) it'll give you hopefully enough horizontal space to add the 2nd roost.
I agree that you wouldn't want to take away run space but as a general rule adding height doesn't add usable space for more birds. Even laddering roosts still requires that you keep them 14-16" apart horizontally (partially so they can't pick on each other, and partially so they don't poop bomb...
They will not roost if they cannot do so safely. The 9 total birds need about 9' of roost space. Expecting them to share a single 4' roost (or a 2nd roost closer than 14-16" from the first) would be like expecting you to share a twin sized bed with 2 random strangers.
Is there anything at all...