Several of you mention the right hand in The Butterfly and this is a good point, though I am afraid with an upright the risk is depressing the keys and no sound coming. The same applies for The Angel.
I completely understand your problem (from direct personal experience), but I do suggest that you won't have that problem on a better upright. The upright I practice on (apart from that it needs tuning) is a good instrument and I have no problem producing a consistent and even pp (in fact, I often find it easier than on some grands), but there are some uprights where it's really hard to go below mf without the risk you mention above, of no sound. They simply aren't very good pianos, or have been badly maintained.