Question n°1: it it true that it can’t be installed, please?
Actually I think it can. The OSPi 1.4 enclosure does not fully expose the microUSB port, but you can always use a dremel or even a diagonal cutter to enlarge the opening so that you can insert a micro USB cable through the enclosure.
Question n° 2: I wonder if it can work as UPS just for the Raspberry Pi.
Looks fine. It’s not clear to me whether this is a switching regulator or a simple linear regulator. If it’s linear, the efficiency will be pretty bad and it will dissipates a lot of power in heat.
Question n°3: which energy source would be used by the Raspberry Pi, please?
I have not checked the Raspberry Pi schematic so I don’t know how it works when you have multiple power sources. My guess is that it has internal mosfets to select a single power source if multiple are present. But in the worst case, you can always use a Schottky diode connected in series with one power source to avoid two power sources competing with each other.