The schematic in post #1 looks like the principle circuit of a zener barrier for intrinsic safety. Its design isn't based on nominal values for e.g. zener voltage or fuse current. You rather need to consider maximal values that can occur under certain defined conditions, e.g. maximal input voltage.
Presumed, fuse and zener diode (actually a threefold parallel circuit according to safety rules) are chosen so, that the fuse protects the zener under the given condtions, e.g. maximal input voltage of 250 V, you get an Vo value (maximal open circuit output) of e.g. 10.5 V. Then R is obtained by impedance matching condition, Po = (Vo/2)²/R. In this case about 92 ohms, a higher nominal value e.g. 100 ohms has to be chosen.
Refer to data sheets of approved zener barriers and safety standards for details.
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Impedance matching condition applies also to the post #3 circuit. The circuit doesn't work for intrinsic safety unless Vin is safely limited to 9V. The fuse is useless then, because the resistor already limits the current to a lower value.